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Salters AS Chemistry - 'exam bashing' thoughts for

Unit PR "The Polymer Revolution" - part of module 2848

Unit map & learning objectives * other backup material * My revision index * My Salters AS homepage * My Salters A2 homepage * EMAIL query?comment

PLEASE REMEMBER, THESE ARE NOT 'STAND ALONE' NOTES, and were designed for my classes for use alongside the Salters resources - Chemical Ideas, Chemical Storylines, Practical Activities-Investigations and the AS-A2 Revision guides all published by Heinemann Secondary Series, to reduce the reading workload and offer a study strategy. From your teacher (not me!), its handy to have the answers to the Chemical Ideas, Storylines Assignments and Activities Questions side by side with the texts and these strategy pages. You haven't time to redo the Q's but a quick read of the Q's and connecting with the official answers is valuable revision - there is too much hit and miss revision from doing past papers in my opinion.

Chemical Storylines PR1 THE START OF THE REVOLUTION"

• 'Natural' polymers are common in nature eg cellulose, proteins etc.

• Some natural polymers adapted into useful products eg cellulose = nitric acid => celluloid

• The first mass produced synthetic plastic was Bakelite made from phenol and methanal (formaldehyde)

• There are two class cases of synthetic polymer 'discovery by accident' ie a case of serendipity. They key point is that a researcher spotted something unexpected was formed and investigated the material's properties.

  • poly(ethene) from a white waxy solid formed in a high pressure benzaldehyde/ethene reaction.

  • poly(tetrafluoroethene), PTFE, formed when F₂C=CF₂ (or C₂F₄)was stored in a metal container, the metal surface catalysed the formation of a white powder.

• What is a polymer?: green box details to know ...

  • be able to describe the meaning or definition of the words: monomer, polymer, polymerisation, elastomer, plastic, fibre.

  • use equations to describe polymerisation processes using full structural, abbreviated or skeletal formula styles (examples shown)

• There are two main types, or modes of formation, of polymers

  • addition polymers formed by single small monomer molecules (often alkenes) adding in a chain like fashion with only the polymer as a product (addition co-polymers are formed by mixing two or more monomers). The process is called polymerisation, in these cases often to form poly(alkenes).

  • condensation polymers are usually formed from two different monomers and on polymerisation small molecules are eliminated when they join up 

Activity PR1  "Some important polymers: introductory data

• All the polymers mentioned in Activity PR1 are addition polymers derived from polymerising alkenes.

• Get to know and use the abbreviations: hdpe, lldpe, ldpe, PVC or old names like polystyrene.

• Appreciate and be able to quote  wide variety of polymers and their properties to suit particular uses (Fig 1).

• Main bulk uses are packaging, containers, tubes and piping.

• The high proportional use of polystyrene and PVC is related to their use in the construction industry and production of non-oil based chemicals like chlorine for PVC from electrolysis of brine (see unit M).

• Thermosoftening plastics can be softened by heat and extruded into piping, injected into moulds or air 'blown' out into bottles.

• hdpe is stronger than ldpe and used for bottles and pipes etc., ldpe is ok for thin film uses.

• Poly(propene) is relatively strong like hdpe and both used a lot for mouldings and is strong enough to be used as textile fibres.

Chemical Storylines PR2 "THE POLY(ETHENE) STORY"

• Discovery of poly(ethene) from a white waxy solid formed in a high pressure benzaldehyde/ethene reaction.

• Another case of serendipity! AND the situation still needs a researcher to spot something unexpected was formed and investigated the material's properties to see if its of any use.

• Later the preparation of the white waxy solid was researched to the point of making enough of it to see if it had useful properties.

• It was noticed that oxygen seem to be an essential catalyst in its formation.

• PE could be heated, moulded into a tough and durable useful electrical insulating material and seemed an excellent replacement for rubber.

• It was tough and durable and chemically unreactive for the mass production of bowls, bottles etc. but did lead to the view of 'cheap and nasty' when used to replace good natural materials.

• PE was one of the first synthetic polymers in the relatively new science of making large molecules ie polymers.

• PE is the simplest organic polymer since it is just made up of -C-C-C- bonds in a chain and C-H bonds off the chain.

• PE has an empirical formula of CH₂ still behaves like an alkane since burns and is unreactive towards most chemical reagents eg acids or alkalis.

  • Note: the empirical formula for a single monomer, and resulting addition polymer, are the same.

• It exhibits typical polymer molecular properties such as:

  • mixture of molecules of different molecular masses or chain length,

  • so it is not a pure compound,

  • and does not melt sharply at a particular temperature, but gradually softens as the temperature is raised.

  • its physical properties contrast with similar but smaller pure molecules eg hexane.

• Assignment 1 is a useful Q.

Chemical Ideas 12.2 "Alkenes" 

• Alkenes are a homologous series hydrocarbons with a carbon = carbon double bond. Examples on the separate alkene structure and naming web page

• Non-cyclic alkenes with one double have the homologous series general formula of C_nH_2n

• Two atoms/groups (X) can add to the double bond via single C-X bond so they are called unsaturated molecules.

• Need to be able to draw the structures of alkenes: (i) full structural, (ii) abbreviated structural formulae, (iii) skeletal (see p282-283 , p287 Q's1-4).

• Need to be able to systematically name alkenes ....ene being the important suffix (see p282-283 , p287 Q's1-4).

• The bond angles about the C=C are all 120^o (three groups of electrons around the C's of the C=C), all the rest of the C-C or C-H are 109^o.

• The bonds around the C=C are in a planar arrangement, so ethene is a completely flat molecule (the rest cannot be e.g. with a -CH₃ group attached as in propene).

• THE CHEMICAL REACTIONS OF ALKENES ... which must be known in detail ...

  • TIPS and general points to avoid problems ...

  • know C₂H₄ is ethene and C₃H₆ is propene (but can be the very different cyclopropane!)

  • BUT try to avoid using them in equations! especially as there are structural isomers for all of the products.

  • eg much better to write CH₃-CH=CH₂ + Br₂ ==> CH₃-CHBr-CH₂Br

  • or even full structural formulae

  • Both better than C₃H₆ + Br₂ ==> C₃H₆Br₂ (which has at least 4 dibromo isomers! work em' out!)

  • In the equations below assume R can be H, alkyl (e.g. -CH₂CH₃) or aryl (e.g. C₆H₅-) and for brevity, abbreviated, but unambiguous, structural formulae are used.

  • All the organic products are usually saturated compounds

• The first four reactions are electrophilic addition reactions.

  • An electrophilic reagent is one that is electron deficient (neutral molecule or positive ion)

  • that will accept an electron pair from an electron rich system eg the double bond in alkenes,

  • and in doing so, the electrophile forms a single bond with one of the carbon atoms of the double bond.

   

• 1. Electrophilic addition of bromine

  • Non-aqueous, either direct element addition or in organic non-polar solvent

  • R₂C=CR₂ + Br₂ ==> R₂CBr-CR₂Br ... the product is a dibromoalkane

  • Technical points and comments to consider ...

    • used as test for alkenes: orange of bromine to colourless dibromo product

    • bromine molecule becomes polarised on collision e.g. ^d+Br-Br^d- to become an electrophile via the ^d+Br

    • note the way the mechanism is laid out and watch the arrows (a full arrow means an electron pair is involved in the 'electronic shift') and there are two steps in the reaction mechanism

    • electrophilic - term used because attacking reagent is an electron acceptor

    • addition - term used because bromine molecule adds to the alkene

     

• 2. Electrophilic addition of bromine in aqueous solution ...

  • Using aqueous solution makes the main product a bromo...alcohol

  • R₂C=CR₂ + Br₂ + H₂O ==> R₂CBr-CR₂OH + HBr

  • Technical points and comments to consider ...

    • the 1st step is the same as in reaction 1. but ...

    • the presence of water changes what is added to the 2nd (d⁺) carbon atom

    • resulting in the formation of an alcohol

    • the 2nd Br (as Br^-) is then associated with the proton H⁺ to form hydrogen bromide.

• 3. Electrophilic addition of hydrogen bromide

  • usually concentrated aqueous solution is used to form a bromoalkane (revise naming from unit A)

  • R₂C=CR₂ + HBr ==> R₂CH-CR₂Br

  • Technical points and comments to consider ....

    • H-Br is a permanently polarised molecule and in fact forms H⁺ (the electrophile) and Br^- ions in water

    • think of as adding a H⁺ instead of a Br^d+ as in reactions 1. and 2.

    • followed by the addition of Br^- as in reaction 1.

    • so the mechanism could be easily constructed

    • the reaction can still be used as test for alkenes because the product is again colourless

   

• 4. Electrophilic addition of water

  • usually via acid catalyst to form an alcohol

  • R₂C=CR₂ +  H₂O ==> R₂CH-CR₂OH

  • Technical points and comments to consider ...

    • the addition takes place in several stages, the proton (the electrophile) adds to the double bond first

    • with sulphuric acid, an intermediate addition compound is formed which then reacts with water to form the alcohol and sulphuric acid (a hydrolysis reaction - a reaction in which something reacts with water to give at least two products)

    • in industry phosphoric acid adsorbed on silica is used, at the higher pressure of 60 atm pressure and 300^oC, and the mechanism is similar.

   

• 5. Addition of hydrogen - called a hydrogenation reaction

  • eg via nickel catalyst to form a saturated alkane

  • R₂C=CR₂ + H₂ ==> R₂CH-CHR₂

  • Technical points and comments to consider ... 

    • the mechanistic details needed are outlined in CI section 10.4

    • this is an important reaction in industry e.g. converting unsaturated fats into saturated fats ie vegetable oils into margarine (which is 'harder' as the saturated product has a slightly higher softening point, which consumers find more 'spreadable'!)

    • 'polyunsaturates' are fat molecules with several double bonds in the carbon chain (a mono unsaturated fat will have one C=C)

• 6. Addition polymerisation (catalyst required) ...

  • Technical points and comments to consider ...

    • free radical chain reaction when catalysed with oxygen or peroxides (revise ideas from unit A)

    • the alkene molecules add together to form the polymer chain (with no other product)

    • be able to write out for poly(ethene) an ...

      • initiation step ... peroxide molecule splits by homolytic bond fission to give two radicals R-O^.

      • propagation step ... an alkene adds to the radical to form R-O-CH2-CH^. to which another alkene molecule can add, BUT there the product is still a very reactive free radical and so addition can continue until ...

      • a termination step occurs ... although of low probability, eventually two free radicals meet and combine - end of that particular chain

      • note the use of half-arrows to show a single electron shift

      • however the propagation step isn't always as simple as you might think ...

      • one problem is that the free radical can interact with the main -CH₂-CH₂-CH₂- chain to create a branch ('back biting')

      • this produces a more 'tangled' low density poly(ethene), cheap, but not as strong as a more linear poly(ethene).

   

Chemical Ideas 5.3 "Forces between molecules: temporary and permanent dipoles" 

• This section contains the most fundamental theory of intermolecular forces you will deal with, but is also one of the most abstract at first.

• SOME REMINDERS and posed Question:

  • All ionic compounds are solid at room temperature and pressure (RTP) due to strong forces between the particles in the giant ionic lattice.

  • Relatively strong covalent bonds hold the atoms together in molecules.

  • BUT the forces between molecules are weak - the so-called intermolecular forces.

  • SO how do theses forces arise? What determines the strength of these forces? and therefore whether a 'simple' molecular substance (ie not a giant covalent lattice) is a gas, liquid or a solid at RTP.

  • We should also consider the energy changes at the  particle level in changing from solid => liquid => gas. (Fig 14).

  • The stronger the forces, the more 'thermal' energy is needed to weaken the inter particle forces to melt or boil a substance and so the following theories explain patterns in melting and boiling points.

• ALL the forces concerned are electrical charge in nature due to the formation of dipoles d+ and d- which will orientate themselves to give an overall greater attraction of d+...d-, rather than d+...d+ or d-...d- repulsion in a given situation.

• There are three types of dipole (a)-(c) described below.

  • (a) Permanent dipole

    • Polar bonds originate when the two atoms constituting a bond have a different electronegativity (the ability of an atom to attract electron charge clouds towards it in a covalent bond situation). So the bonding pair of electrons is unequally shared creating the d+ and and d- dipole.

    • The most electronegative atom carries the ^d- and the least electronegative element the ^d+

      • e.g. H^d+-Cl^d- for HCl or H^d+-O^d2--H^d+ for H₂O. 

    • The presence of a polar bond usually results in the molecule being polar ie having an overall molecular dipole (but see CCl₄ later).

  • (b) Instantaneous or temporary dipole

    • Molecules which have no permanent dipole usually have no polar bonds e.g. Cl₂ (both atoms the same) or CH₄ (C and H similar electronegativity).

    • BUT these molecules are still attracted to each other to form a liquid or solid due to the formation of a temporary or instantaneous dipole.

    • This dipole originates from the random fluctuations in the electron charge clouds as the electrons move around the nucleus.

    • So at any given instant, the electron charge is never symmetrically distributed and local d+ and d- dipoles are formed.

    • Remember all particles will form this kind of dipole.

  • (c) Induced dipole

    • If an unpolarised molecule is next to a polarised molecule a dipole can be induced as the polarised molecule will attract or repel the electron charge in the neighbouring molecule.

    • The original polarised molecule can have a permanent or transient/temporary dipole (the former having the bigger effect).

    • e.g. H^d+-Cl^d- Cl-Cl leads to H^d+-Cl^d-...Cl^d+-Cl^d-

• There are three types of dipole interactions which contribute to intermolecular forces.

• (1) Permanent dipole - permanent dipole [(a)-(a) interaction, the strongest]

  • This produces the strongest intermolecular forces between permanently polar molecules like hydrogen chloride or water etc.

  • It can lead to unusually high boiling points compared to non-polar molecules e.g. CH₄ M_r = 16 bpt -162^oC and H₂O M_r = 18 bpt = 100^oC (this is an extreme example due to so-called 'hydrogen bonding' see CI 5.4 later).

  • On time averaged basis the repulsion of like charges is outweighed by unlike charges attracting (and this applies to all dipole interactions).

  • Bond polarity depends on the difference in electronegativity of the two atoms but the dipole of the molecule also depends on the shape.

  • e.g. ^d+H₃C-CCl₃^d- is a very polar molecule BUT CCl₄ is not a polar molecule because the effect of the four C^d+-Cl^d- dipoles of the polar bonds are 'cancelled out' because of the tetrahedral symmetry of the molecule!

  • Also bear in mind that interactions (2) and (3) also apply to molecules with polar bonds ie they also contribute to the overall intermolecular attractive force [but in effect (1) > (2) > (3)]

  • The intermolecular forces increase with increase in bond polarity (eg C-Cl > C-Br) and if the dipoles can get closer together (eg in plastics if the polymer molecules are more aligned, or in so-called 'hydrogen bonding' - see CI 5.4)

• (2) Permanent dipole - induced dipole [(a)-(b) interaction, the next strongest]

  • eg H^d+-Cl^d- Cl-Cl leads to H^d+-Cl^d-...Cl^d+-Cl^d-

• (3) Instantaneous/temporary dipole - induced dipole [(b)-(c) interaction, the weakest]

  • Although the weakest of the interactions, they can be very significant for large non-polar molecules like poly(ethene)

  • This force increases, the greater the number of electrons in the molecule, because the larger volume of electron clouds is more polarisable.

  • eg even the very weak forces between the single atoms of the Noble Gases, increases with atomic number, so the melting and boiling points increase down the group.

  • In the case of non-polar alkanes, the boiling point steadily rises up the homologous series C_nH_2n+2 because the molecule gets steadily larger, with more electron cloud volume, more surface-surface contact possible, so more polarisable causing a steady increase in the intermolecular forces.

  • Unlike (1) and (2), it is often forgotten that this type of interaction applies to all particles.

  • The shape of the molecule can also influence the strength of the interaction, and so produce boiling point differences eg ...

  • for alkane structural chain isomers, the more branched the isomer the more compact it is, the less surface-surface contact, the weaker the intermolecular attractive forces and so the lower the boiling point.

  • The differences are rarely large but not insignificant eg for the alkane isomers of C₅H₁₂ ...

  • CH₃CH₂CH₂CH₂CH₃ [bpt 36^oC] > (CH₃)₂CHCH₂CH₃ [bpt 28^oC] > (CH₃)₄C [bpt 10^oC]

Chemical Ideas 5.5 "The structure and properties of polymers" 

• Only the 1^st part p109-113 on addition polymers is needed for AS module 2848.

• A polymer is a long molecule made up from small molecules called the monomer.

• You can use one monomer to form a -A-A-A-A-A- etc. polymer (eg poly(ethene), PVC).

• Or you can use two monomers to form a -A-B-A-B-A-B-A-B- in strict alternation etc. polymer (eg nylon or polyester - which are condensation polymers, meaning the monomers link up by the elimination of a small molecule like H₂O, not needed in PR, so don't confuse them with addition co-polymers mentioned below).

• Addition polymerisation is when the monomer molecules add together to form a chain (of varied length) and no other product.

• The monomer usually contains a C=C double bond, ie an alkene, polymerising to form a poly(alkene), 'half' of the double bond opens to link to the next monomer unit etc.

• Note the different ways of representing the polymer eg part of the chain or abbreviated showing the repeating unit in () as above

• You can use two alkene monomers to form a co-polymer eg from ethene and propene, but unlike nylon or polyester, the A/B arrangement on average depends on the A/B ratio is randomised along the chain eg -A-B-A-A-B-A-B-B-B-A-B-A-B-A-B-B-A etc.

• Addition polymers can also be made from alkynes eg ethyne H-CC-H or propyne CH₃-CC-H, one of the three bonds of the triple bond opens to allow linking. However, a double bond is left in the main polymer chain eg ethyne forms poly(ethyne) part of which looks like -CH=CH-CH=CH-CH=CH- etc. These are interesting polymers because they conduct electricity, the alternate double-single carbon-carbon bond systems 'merge' in a continuous delocalised electron or conjugate bond system, through which electrons can move under an applied voltage (see Storylines PR6 p103).

• There is huge variety of polymer forms ...

  • Elastomer: polymers which are soft and springy, readily deformed, but spring back to their original shape (eg rubber)

  • Plastic: polymers not as springy as elastomers, tend to stay deformed in the new shape when force applied (especially if heated) eg poly(ethene).

  • Fibre: stronger polymers drawn into thin strands to align the molecules and maximising the intermolecular forces. These can be woven into 'cloth' material eg nylon or bound into a 'rope' form eg poly(propene) climbing rope.

• The properties of polymer such as strength or flexibility, depend on its molecular structure and the resulting intermolecular forces ... some characteristics are outlined below ...

  • Chain length: the longer the chain the stronger, because more there are more possible dipole interactions per molecule.

  • Side groups: groups with polar bonds eg C^d+-Cl^d- or H^d+-N^d- or C^d+=O^d- increase polymer strength from the permanent dipole - permanent dipole interactions, C-OH polar bonds can hydrogen bond with water to increase solubility.

  • Branching: straight unbranched chains can align closely and maximise the intermolecular forces and so the strength of the polymer, but more branching means the chains can't pack as tightly, so these tend to be less strong and more flexible.

  • Sterioregularity: polymer chains pack more closely if the side chain groups are all aligned or orientated in a regular way, so increasing polymer strength.

  • Chain flexibility: hydrocarbon chains are very flexible giving flexible polymers, if the chain can be made more rigid the polymer is stronger

  • Cross-linking: if adjacent polymer chains can be linked by strong covalent bonds called cross-links, the structure becomes much more rigid and stronger (see thermosets below).

• Polymers can be classified into two groups according to their behaviour when heated (Figs 29-30).

  • Thermoplastics are polymers without cross-links between the chains. The intermolecular forces are much weaker than covalent bonds and so they are readily, and considerably, weakened on heating. The polymer chains can move over each other making the material very 'plastic' when heated. On cooling they retain their new shape completely if deformed when hot.

  • Thermosetting polymers have extensive cross-linking via covalent bonds joining adjacent polymer chains together into a 3D giant covalent structure. This prevents the chains moving apart so the shape is retained and the polymer is much more heat resistant. They do not melt but eventually decompose at high temperature into small gaseous molecules and a 'charred' residue. They are strong and hard materials and do not dissolve in solvents.

• Polymer chain length and strength (Fig 31):

  • In general the longer the chain the stronger the intermolecular forces. However until a 'polymer' is a certain length it is a very weak material in terms of tensile strength. Then, within a certain chain length range, the strength steadily increases but finally reaches a limiting value.

  • Two factors cause this rise in strength:

    • Longer chains get more tangled and make the molecular structure more rigid.

    • The longer the chain the greater the possibility of dipole interactions (of any origin) increases per molecule. 

• Crystalline and amorphous polymer regions (usually co-exist in same polymer, see Fig 32):

  • Crystalline regions are where the polymer molecules are lined up together in an regular way and so maximising the intermolecular forces.  Crystalline regions are more abundant with polymers with regular chain structures such as isotactic poly(propene) or without bulky side groups or extensive chain branching eg high density poly(ethene) hdpe.

  • Amorphous regions are where the polymer chains are more randomised and tangled up.

  • The % crystallinity in a polymer is very important in determining its properties. The more crystalline it is, the stronger and less flexible it is.

• Cold-drawing (Fig 33) is a process designed to increase the strength of a polymer. The polymer is stretched without heating, and in the 'neck' section the polymer molecules to line up adjacent to each other and maximise the intermolecular forces. This strengthens the material and is an essential process for producing strong fibres. 

• Questions 1-8 are essential revision.

Activity PR2 "Making poly(phenylethene)" (polystyrene)

• This is good and instructive demonstration, particularly if not seen at GCSE level.

• The questions are 'exam like' and should be revised eg ideas on catalysts, free radicals and polymerisation equation.

Chemical Storylines PR3 "TOWARDS HIGH DENSITY POLYMERS"

• The intermolecular forces holding poly(ethene) molecules together are the weakest of the dipole interactions (transient - induced dipoles). So how can they be maximised by 'controlling' the molecular structure to make the strongest possible PE?

• The early production processes offered some control on the polymerisation process, but it produced low density poly(ethene) ldpe. The ldpe chains are very branched and can't line up easily in a regular way to maximise surface-surface dipole interactions, and taking up more space. This leads to low density and low tensile strength because of lowered intermolecular forces.

• Ziegler developed organ metallic catalysts [eg TiCl₄/Al(CH₂CH₃)₃ mixture] which produced high density poly(ethene) hdpe. hdpe has a very high M_r and little branching, consequently the polymer chains can line up in a regular way to maximise the transient - induced dipole forces. 

• The more dense packing of hdpe makes it more crystalline, stronger and more heat resistant to softening (*). It can be readily moulded into washing up bowls, water tanks and piping, whatever the complexity of the shape eg car petrol tanks to suite any car design. (* it means articles can be heat sterilised like buckets and bed-pans in hospitals)

• Natta developed Ziegler's catalyst method to make poly(propene) in various forms, and since then, as well as these Ziegler-Natta catalysts, a new generation of other steriospecific catalysts called metallocenes are offering even more control eg molecular mass and orientation of side-chain groups. Note the development of new catalysts allows polymer design to advance (Fig 10). Three forms of poly(propene) can be made (in order of decreasing crystallinity and strength, see Figs 8,9,10) ...

  • Isotactic: very regular structure, all the side-chain methyl groups have the same orientation, strong, crystalline and rigid like hdpe. Used in sheet and film form for packaging and fibres for catalysts. Thin films can be made which are more impermeable to air and water, strong and tear-resistant and excellent for food packaging.

  • Syndiotactic: the methyl groups are orientated in alternate positions on the chain, this less regular structure makes the polymer a little less dense and more flexible, so its properties are intermediate between isotactic and atactic poly(propene).

  • Atactic: the orientation of the methyl groups are randomised down the chain producing a much more amorphous form, less dense and more soft and flexible than isotactic or syndiotactic. Used for making roofing materials, sealants and other weatherproof coatings.

• Assignments 2 and 3 are good exam revision.

Activity PR PR3 "Using spaghetti to model polymer structure"

• A fine pan of spaghetti gives a good picture of Fig 32 Chemical Ideas 5.5 p112

Chemical Storylines PR4 "THE TEFLON MAN"

• Discovered by accident, metal surface of canister catalysed polymerisation.

• The equation for tetrafluoroethene ==> poly(tetrafluoroethene)

• The C-F bonds are polar, all the covalent bonds (C-C or C-F) are very strong, so it is a thermally stable polymer and doesn't oxidise easily either eg used as frying pan surface

• Very good 'anti-stick' properties (frying pan mention again!), excellent electrical insulator, highly resistant to chemical attack

• Used in Gore-tex but there was an initial perspiration problem! Its a hydrophobic material but can be stretched to a porous form which allows water vapour through but not liquid, so it can 'breathe'.

Chemical Storylines PR5 "DISSOLVING POLYMERS"

• The development of poly(ethenol), this is based on the fictitious (not stable) CH₂=CHOH, its made indirectly from another polymer, poly(ethenyl ethanoate), based on the ester CH₂=CHOCOCH₃ which does exist!

• The poly(ethenyl ethanoate) can be reacted with methanol to free some of the -OH groups (Fig 17 p102, methyl ethanoate is also formed as a side-product).

• Note the skeletal formula representation, convert to structural formula in repeating unit style?

• The -OH groups can hydrogen bond to water, making this polymer soluble.

• One of poly(ethenol)'s applications is soluble laundry bags in hospitals to avoid contact and risk of infection. The dirty linen is contained until the bag dissolves in the washing process.

• Assignment 4 is a really good exam thinking Q.

Chemical Ideas 5.4 "Forces between molecules: hydrogen bonding"

• The unusual properties of water demand an explanation eg relatively high melting/boiling points and enthalpies of vaporisation, for such a small molecular mass and the density of the solid being less than the liquid.

• The high boiling point and dH_vap of water contrast with the expected steady increase of them (from an expected low value for water) for increasing molecular mass along a series of hydrides (hydride is a compound with hydrogen, see Figs 20 and 21)

• Also note the equally anomalous behaviour of ammonia ('hydride of nitrogen') and hydrogen fluoride ('hydride of fluorine').

• This suggests there is something special about the intermolecular forces for molecules with a H-F, H-O or H-N bond in them. The explanation lies in the concept of the strongest intermolecular forces called hydrogen bonding.

  • NOTE THAT IT IS NOT A REAL COVALENT OR IONIC BOND, its just the strongest form of permanent dipole ... permanent dipole interactions, and is given a the special name 'hydrogen bonding'.

  • There is an interesting comparison of energy values for different types of attraction in table 7 p105.

• The special nature of hydrogen bonding involves three features which all contribute to increasing intermolecular forces compared to many other molecules....

  • A large permanent dipole based on a highly polar H^d+-X^d- bond where X is usually N, O or F (three of the four most electronegative elements), these combinations give a relatively large difference in electronegativity between the atoms of the bond.

  • the tiny H atom (proton) can get very close to atoms of an adjacent molecule,

  • a lone pair of electrons on the ^d- atoms N, O or F and can line with the ^d+ of the hydrogen atom.

• Figs 22 to 25 on pages 104-105 all illustrate these ideas, but note ...

  • only one hydrogen bond per molecule for HF and NH₃ (explanations given)

  • but two hydrogen bonds per molecule for water, this leads to quite a strong 3D covalent bond and hydrogen bond network (see below).

• Compared to non-polar molecules, molecules exhibiting hydrogen bonding e.g. alcohols, organic carboxylic acids, amines, amides (including amino acids and proteins), may show relatively high boiling and melting points, high viscosities, and contribute to the 3D secondary and tertiary structures of proteins and ice [in a later unit you will look at nylon fibre structure and the solubility of poly(ethenol) is mentioned in Storylines PR5].

• The properties of water, interpreted through the influence of hydrogen bonding are given in detail on pages 106-107 Figs 27 and 28.

  • Specific heat capacity is energy required to increase the temperature of a material in terms of Jg^-1K^-1. It is higher than expected because more energy is needed to overcome hydrogen bonding before the thermal kinetic energy of the particles can be increased. Some of the energy is used to break or weaken hydrogen bonds rather than increase the KE of the particles, which is what increase in temperature amounts to.

  • The melting/boiling points and enthalpy of vaporisation (Jmol^-1) are much higher than expected for the same reasons given above.

  • The low density of ice compared to water is explained by the open crystal structure produced by the 'tetrahedral' arrangement of two covalent bonds and two hydrogen bonds around each water molecule.

    • When ice melts, enough of the hydrogen bonds are weakened to allow the water molecules to get on average closer to each other and so raising the density.

    • For both ice and water you expect the density to steadily fall with increase in temperature due to increased KE/thermal energy of the particles moving them further apart.

    • This is so for ice, but water shows a maximum in density at 4^oC, why?

    • The reasons are (i) hydrogen bonds persist in clumps of molecules, as these break down the density rises, but (ii) increased temperature causes expansion of the 'freed' molecules. Upto 4^oC factor (i) outweighs (ii) and after 4^oC (ii) outweighs (i), get it?

    • Finally, consider how this, not too complex molecular explanation, helps the much more complex molecular pond-life to survive the winter and who want's to be reminded of burst water pipes caused by the formation of too many hydrogen bonds! You do! It might be on the exam!

CI 13.2 "Alcohols and ethers" (revision)

• Revision of the structure and naming of alcohols and their basic physical properties.

  • see summary of alcohols on DF notes

CI 13.4 part a "The -OH group in alcohols, phenols & carboxylic acids"

(see also the new activity PR5.5)

• From CI 13.3 and this 13.4 section, make sure you can recognise members of the following homologous series:

  • alcohols, phenol (-OH attached directly to a benzene ring), aldehydes & ketones, and carboxylic acids

  • see homologous series & functional group page

• Be able to describe and explain characteristic chemical properties of alcohols, writing/interpreting equations, reagents used, observations etc. including:

  • be able to recognise, and distinguish the different structures of primary, secondary and tertiary alcohols (p310)

  • oxidation of alcohols to carbonyl compounds and carboxylic acids with aqueous acidified H₂SO_4(aq) potassium dichromate(VI) solution K₂Cr₂O_7(aq) 

    • note the colour change from orange Cr₂O₇^2- to green Cr³⁺.

• Be able to describe the following properties of aldehydes and ketones

  • recognise as group of carbonyl C=O compounds,

  • know the structure difference between aldehyde and ketone

  • know their formation from oxidising alcohols (primary alcohol ==> aldehyde ==> carboxylic acid, secondary alcohol ==> ketone, tertiary not readily oxidised - if so, carbon chain broken giving H₂O, CO₂ or lower RCOOH etc.

• The oxidation of alcohols using moderately conc. sulphuric acid and potassium dichromate(VI) is an important organic synthetic method to know.

  • The colour change from orange [dichromate(VI) Cr₂O₇^2- (Cr +6 ox. state]  to green (chromium(III) Cr³⁺ (Cr +3 ox. state)] is an important indicator that the oxidation reaction has taken place.

• You must know the sequences and the relevant molecular structure changes:

  • primary alcohol => aldehyde => carboxylic acid ( end!) *

    • RCH₂OH => RCHO => RCOOH

      • If done under reflux you will tend to get the carboxylic acid because aldehydes are more easily oxidised than the original primary alcohol! However, it is technically possible to rapidly distill off the aldehyde the moment it is formed and prevent the 2nd oxidation stage to form the carboxylic acid.

  • secondary alcohol => ketone ( end!) *

    • R₂CHOH => R₂C=O

  • tertiary alcohol R₃COH ( end!) *

    • * ketones, tertiary alcohols and carboxylic acids are (usually) reasonably stable against further oxidation since strong C-C bonds must be broken to form lower carboxylic acids, carbon dioxide and water etc. This 'breakdown' reaction is of no synthetic use at all.

• The importance of recognising the type of alcohol (prim/sec/tert) is important in interpreting the results of the investigation.