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organic reaction mechanismsDoc Brown's GCE Chemistry  Revising Advanced Level Organic Chemistry

A Level Revision Notes PART 10 Summary of organic reaction mechanisms

A mechanistic introduction to organic chemistry and explanations of different types of organic reactions

Part 10.4 Halogenoalkanes

Part 10.4 HALOGENOALKANES - introduction to the mechanisms of halogenoalkanes (haloalkanes, alkyl halides). Nucleophilic substitution by water and nucleophilic substitution by the hydroxide ion. These revision notes include full diagrams and explanation of the nucleophilic substitution reaction mechanisms of halogenoalkanes (haloalkanes) and the 'molecular' equation and reaction conditions and other con-current reaction pathways and products are also explained when halogenoalkanes react with water and alkalis to give alcohols on hydrolysis. Water, amines and hydroxide ion are typical electron pair donating nucleophiles that can attack a partially positive carbon atom of a carbon-halogen bond e.g. C-Cl, C-Br and C-I.


10.4 HALOGENOALKANES (old names 'haloalkanes' or 'alkyl halides')

10.4.1 Introduction to halogenoalkane reactivity

  • Halogenoalkanes owe their reactivity, especially compared to the unreactive alkanes, to two principal reasons.

  • R3C-X = halogenoalkane/haloalkane/alkyl halide/halogenated alkane etc. X = halogen e.g. Cl, Br or I

  • The carbon-halogen bond is polar, Cδ+-Xδ- due to the difference in electronegativity between carbon and the halogen. The  Cδ+ carbon is then susceptible to nucleophilic attack by electron pair donor neutral molecules (e.g. the nucleophiles :NH3, H2O:) or ions (e.g. :OH-, -:CN).

  • The carbon-halogen bond is usually the weakest bond in the molecule and significantly weaker than the carbon-carbon or carbon-hydrogen bonds.

    • Average bond enthalpies/kJmol-1: C-C 348, C-H 412, both relatively high requiring high activation energies for reaction.

    • Average bond enthalpies/kJmol-1: C-Cl 338, C-Br 276, C-I 238, generally lower resulting in lower activation energies.

      • Even the lowering of the bond enthalpy by 10kJ from C-C to C-Cl, combined with the polarity of the C-Cl bond, makes all the difference when comparing alkane and halogenoalkane reactivity.

  • A comparison of aliphatic halogenoalkanes and aromatic halides is dealt with in FURTHER COMMENTS.

  • IMPORTANT NOTE on structure classification

    • In the mechanism diagrams you will see part of the molecular structure shown as R3C

    • PLEASE do not assume this means a tertiary (tert) halogenoalkane (haloalkane).

    • R3C- is used repeatedly to minimise the number of graphic images needed.

    • In general a halogenoalkane (haloalkane) has the structure R3C-X where R = H, alkyl or aryl.

    • A primary halogenoalkane (haloalkane) can be shown as RCH2-X where R = H, alkyl or aryl.

    • A secondary (sec) halogenoalkane (haloalkane) can shown as R2CH-X where R = alkyl or aryl.

    • A tertiary (tert) halogenoalkane (haloalkane) can be shown as R3C-X where R = alkyl or aryl.

 


 

10.4.2 Nucleophilic substitution of halogenoalkane by hydroxide ion (hydrolysis with OH-)

Organic synthesis of alcohols from halogenoalkanes (haloalkanes, alkyl halides) by reaction with sodium/potassium hydroxide

  • What is the reaction mechanism for the hydrolysis of halogenoalkanes/haloalkanes?

  • e.g. for the reaction hydrolysis with NaOH(aq): [see mechanisms 1, 2, 33 and 34 below]

  • R3C-X + OH- ==> R3C-OH + X-

  • The halogenoalkane is usually refluxed with aqueous sodium hydroxide, NaOH(aq), but some RX molecules are reactive enough to hydrolyse when just mixed with water (see further down).

  • The reaction can proceed by two different mechanisms, which can occur simultaneously!, and are sometimes referred to as SN1 'unimolecular' and SN2 'bimolecular', BUT these 'molecular' terms are based on kinetic studies of the reaction and refer to the overall order of the reaction (see reaction kinetics). The vast majority of reaction steps occur via bimolecular collisions, even if its only with the solvent (as in the case of step (1) of the SN1 mechanism below), so beware of terminology.

  organic reaction mechanisms

mechanism 1 - nucleophilic substitution of a halogenoalkane by hydroxide ion (SN1 'unimolecular' via carbocation)

  • SN1 unimolecular, a two step ionic mechanism via carbocation formation [mechanism 1 above]

    • In step (1) the Cδ+-Xδ- polar bond of the halogenoalkane splits heterolytically to form a carbocation and a free halide ion (X-, e.g. chloride or bromide) and this is a reversible reaction. The C-Cl bond is most likely to break because it is a weaker bond than the C-C or C-H bond AND breaks heterolytically because of the big difference in electronegativity between carbon (2.1) and chlorine (3.0) giving a polar Cδ+-Clδ- bond. So the C-Cl bonding pair of electrons leaves with the Cl atom as the Cl- ion.

    • In step (2) the hydroxide ion is a negative electron pair donor and rapidly combines with the carbocation, forming the C-O bond in the alcohol product. The hydroxide ion is the nucleophile.

    •  Step (1) is the rate determining step with the much larger activation energy (see reaction profile diagram 45)

    • This mechanism is most likely with tertiary halogenoalkanes.

    • Primary halogenoalkanes tend to react by the SN2 mechanism NOT involving a carbocation.

    • Secondary halogenoalkanes react via both mechanisms at the same time! The relative rate order for the SN1 mechanism is tert > sec > prim i.e. following the pattern of decreasing stability of the carbocation (see extra discussion points).

Three diagram 'styles' are shown below for the SN2 bimolecular mechanism that does NOT involve a carbocation.

style (a) organic reaction mechanisms

mechanism 2 - nucleophilic substitution of a halogenoalkane by hydroxide ion (SN2 'bimolecular')

style (b) organic reaction mechanisms

mechanism 34 - nucleophilic substitution of a halogenoalkane by hydroxide ion (SN2 'bimolecular')

style (c) organic reaction mechanisms

mechanism 33 - nucleophilic substitution of a halogenoalkane by hydroxide ion (SN2 'bimolecular')

  • SN2 'bimolecular', a one step bimolecular collision mechanism [mechanisms 2, 34 and 33 above]

    • The Cδ+-Xδ- bond is polar because of the difference in electronegativity between carbon (2.1) and chlorine (3.0), so the electron rich nucleophile, the hydroxide ion, attacks the slightly positive carbon.

    • The nucleophile acts as an electron pair donor (Lewis base) to bond with the Cδ+ carbon to make the C-O bond in the newly formed C-OH alcohol group.

    • Simultaneously the chlorine atom is ejected, taking with it the C-X bond pair, so forming the chloride ion on expulsion.

    • This mechanism is most likely with primary halogenoalkanes. Tertiary halogenoalkanes tend to react by the SN1 mechanism involving a carbocation, secondary halogenoalkanes react via both mechanisms (see extra discussion points).

  • Diagram styles and explanation:

    • Style (a) Does not show the 'activated complex' or 'transition state' at the point where the hydroxide ion ('incoming') and the chloride ion ('outgoing') are 'half-bonded' to the central carbon atom of the functional group.

    • Style (b) Shows a simplified version of the 'activated complex' or 'transition state'. The blue .... represent 'half-bonds' (not weak inter-molecular forces) in the sense that the hydroxy/hydroxide group is 'coming in' and the chloro/chloride group is 'going out'!

      • The change can also be represented as a simple reaction progress-profile diagram 41.

      • Note that 'activated complex' or 'transition state' is not the same as an intermediate like a carbocation which is a definite entity in its own right, however short its lifetime.

    • Style (c)  Shows the reaction in terms of the stereochemistry. This is not a particularly important 'style' unless the original halogenoalkane is chiral (see note on chirality below).

Also read the FURTHER COMMENTS for this reaction

  • Direct hydrolysis with H2O: [mechanisms 10 and 35 below]

  • R3C-X + 2H2O ==> R3C-OH + X- + H3O+

organic reaction mechanisms

mechanism 10 - nucleophilic substitution of a halogenoalkane by water (SN1 unimolecular via carbocation)

  • SN1 unimolecular, a two step ionic mechanism via carbocation formation. [mechanism 10 above]

    • In step (1) the halogenoalkane splits heterolytically to form a carbocation and a free halide ion (e.g. chloride or bromide) and this is a reversible reaction. The C-X bond is most likely to break because it is a weaker bond than C-C or C-H AND breaks heterolytically because of the big difference in negativity between carbon (2.1) and chlorine (3.0) giving a polar Cδ+-Xδ- bond. So the C-Cl bonding pair of electrons leaves with the Cl atom as the Cl- ion.  Step (1) is the rate determining step (rds) and the rate effectively only depends on the halogenoalkane concentration.

    • In step (2) the water is an electron pair donor and rapidly combines with the carbocation, forming the C-O bond in the protonated alcohol. The water molecule is the nucleophile

    • In step (3) a water molecules rapidly removes a proton to leave the free alcohol product.

    • The overall change can also be represented as a reaction progress-profile diagram 42.

    • This mechanism above is most likely with tertiary halogenoalkanes.

    • Primary halogenoalkanes tend to react by the SN2 mechanism NOT involving a carbocation, though very slow with water if it reacts at all. Secondary halogenoalkanes tend to react via both mechanisms at the same time (see extra discussion points).

organic reaction mechanisms

mechanism 35 nucleophilic substitution of a halogenoalkane by water (SN2 bimolecular)

  • SN2 'bimolecular', a two step bimolecular collision mechanism but NOT involving a carbocation. [mechanism 35 above]

    • Step (1): The Cδ+-Xδ- bond is polar because of the difference in electronegativity between carbon (2.1) and chlorine (3.0), so the electron rich nucleophile, the water molecule, attacks the slightly positive carbon. The nucleophilic water acts as an electron pair donor (Lewis base) to bond with the 'delta positive' carbon to give the C-O bond of the protonated alcohol. Simultaneously the chlorine atom is ejected, taking with it the C-X bond pair, so forming the chloride ion on expulsion. Step (1) is the rate determining step (rds) and the rate effectively only depends on the halogenoalkane concentration.

    • In step (2) another water molecule rapidly accepts the proton from the protonated alcohol to leave the free alcohol product.

    • The reaction profile would be similar to diagram 45 

      • BUT the intermediate is [R3COH2]+ NOT an R3C+ carbocation.

    • This mechanism is most likely with primary halogenoalkanes, but very slow if at all with water, much faster with the hydroxide ion, a more powerful nucleophile (negative ion as well as an electron pair donor). Tertiary halogenoalkanes tend to react by the SN1 mechanism involving a carbocation, secondary halogenoalkanes react via both mechanisms (see extra discussion points).

  • FURTHER COMMENTS on these halogenoalkane (haloalkane) nucleophilic substitution reactions

    • Comparison of aliphatic halogenoalkane and aromatic halide hydrolysis.

      • Halogenoalkanes hydrolyse much more readily than aryl halides which are aromatic compounds with a halogen atom directly bonded to the benzene ring. The carbon-halogen bond is stronger and less polar in aromatic compounds compared to halogenoalkanes. This is because in the aromatic halogen compounds there is some overlap/delocalisation of the lone pairs of electrons of the chlorine with the electrons of the benzene ring. This makes the aromatic ring C-Hal bond stronger and less easy to break, hence less polar and less susceptible to nucleophilic attack.

        • chloromethylbenzeneHowever, if the halogen is in an alkyl side chain off the benzene ring, hydrolysis will readily take place.

          • e.g. refluxing chloromethylbenzene (phenylchloromethane, above left) with aqueous/ethanolic sodium hydroxide gives phenylmethanol.

          • C6H5CH2Cl + NaOH ==> C6H5CH2OH + NaCl

        • chloro-2-methylbenzene or chloro-3-methylbenzene or chloro-4-methylbenzene Whereas hydrolysis of any of the three other isomeric aromatic halogen compounds, namely chloro-2/3/4-methylbenzene (above left) where the Cl atom is attached to the ring, is very difficult to achieve to produce a phenol (where the OH hydroxy group is directly attached to a benzene ring) by hydrolysis.

        • ClC6H4CH3 + NaOH ==> HOC6H4CH3 + NaCl

        • is not impossible, BUT, NOT EASY! though it can be brought about at higher temperatures and high pressures than normal reflux conditions in the school laboratory, but in an 'industrial' sealed reaction vessel.

    • Rate of reaction, OH- versus H2O:

      • Only applies to the SN2 mechanism: Irrespective of the structure of the halogenoalkane, the hydrolysis will be faster with aqueous sodium hydroxide than just water because the hydroxide ion is a more powerful nucleophile. Although they are both electron pair donors, the hydroxide ion carries a full negative charge compared to the electrically neutral water molecule.

      • The rate determining formation of the carbocation in the SN1 mechanisms means the rate is not affected by using alkali or just water (read about the kinetics below).

    • Reaction kinetics: The possibility of two reaction mechanisms has consequences for the rate expressions when the rates of halogenoalkane (RX) nucleophilic substitution reactions are studied.

    • The SN1 mechanism is referred to as a 'unimolecular', despite it being a two/three step mechanism of bimolecular collisions, because the rate is only dependent on one reactant, the R3C-X is shown in step (1), but it still has to collide with the solvent!

      • [see mechanism 1 or mechanism 10 and reaction profile 42]

      • Experimental results produce the overall 1st order rate expression: rate = k1[RX] 

      • This is because the activation energy of the 1st step, forming the carbocation by heterolytic bond fission, is so high, that the speed is relatively low, so step (1) alone determines the speed of the reaction. This is referred to as the rate determining step (or rds in shorthand!). Step (2) has a much lower activation energy and is much faster (see reaction profile diagram 42). You would register zero order for the order of reaction with respect to sodium hydroxide (or more specifically, the hydroxide ion concentration).

    • The SN2 mechanism is referred to as a 'bimolecular'.

      • [see mechanisms 2/33/34 or mechanism 35 and reaction profile 41]

      • Experimental results produce the overall 2nd order rate expression: rate = k2[RX][OH] 

      • This is because it is a one step mechanism involving the bimolecular collision of the two reactant molecules/ions (see also reaction profile diagram 41). The rate depends on both the halogenoalkane and hydroxide ion concentrations (individually, 1st order with respect to both). 

    • The relative reactivity of the C-X bond, where X = F, Cl, Br or I.

      • In general the reactivity order is C-I > C-Br > C-Cl > C-F.

      • This is due to the decrease in bond enthalpy as the halogen atom radius increases the bond gets longer and weaker, i.e. easier to form the carbocation in the SN1 mechanism or easier to release a X- ion from the 'activated complex' in the SN2 mechanism.

        • Average bond enthalpies/kJmol-1: C-F 484, C-Cl 338, C-Br 276, C-I 238

      • This bond strength factor overrides any increase in bond polarity, which on face value, since the order of becoming less polar is: C-F > C-Cl > C-Br > C-I, which would suggest an increase in susceptibility to nucleophilic attack at the delta positive carbon, but that's not what is observed!

        • So bond enthalpies override the bond polarity trend.

    • Carbon chain structure and relative reactivity.

      • Generally speaking the reactivity order is tert > sec > prim halogenoalkane, but unfortunately it isn't quite that simple!

        • For the SN1 mechanism the reactivity trend is tert > sec > prim halogenoalkane.

          • The reason for this is that alkyl groups have a small electron charge donating inductive effect (+I effect). Its actually the positive charge on the carbon (from the C-X bond) attracting the electron charge of the alkyl group(s) which helps stabilise the carbocation by 'spreading' the charge and lowering the potential energy of the intermediate carbocation.

          • So a typical reactivity series is (CH3)3CBr > (CH3)2CHBr > CH3CH2Br > CH3Br

            • because the order of carbocation stability would be ...

            • (CH3)3C+ > (CH3)2CH+ > CH3CH2+ > CH3+

            • where 3, 2, 1 and 0 methyl groups are available to stabilise the carbocation.

            • The reason for this carbocation stability trend is often referred to as an increasing +I or inductive effect of an increasing number of alkyl groups. The alkyl group is considered to 'donate' electron charge to the positive carbon atom to stabilise the carbocation. However, it is better considered that the positive charge on the positive carbon attracts the greater electron clouds of the alkyl groups compared to hydrogen atoms and so spreads the charge more widely stabilising the carbocation, hence the more alkyl groups on the functional group carbon, the more stabilisation can occur.

          • SN1 is also favoured by polar solvents like water or ethanol, which help stabilise the carbocation by solvation.

        • For the SN2 mechanism the reactivity trend is prim > sec > tert halogenoalkane because of steric hindrance, that is alkyl groups get in the way of the incoming nucleophile such as the hydroxide ion or water molecule. It is usually very slow with water, but no problem if the R3C-X is warmed/refluxed with aqueous sodium hydroxide, though some alcohol can be added to increase the solubility of R3C-X to increase the rate of hydrolysis.

        • The SN1 mechanism  is favoured by tertiary halogenoalkanes because of the greater stability of tertiary carbocations formed.

        • Secondary halogenoalkanes often react by either the SN1 or SN2 mechanisms simultaneously.

        • Primary halogenoalkanes tend to react by the SN2 mechanism, because on heterolytic C-X bond fission, they would form the least stable primary carbocations.

        • For a given halogen, overall tertiary halogenoalkanes tend to be the most reactive and tend to undergo nucleophilic substitution via the SN1 carbocation mechanism and primary halogenoalkanes tend to be the least reactive and react via the SN2 mechanism BUT it is a very complicated situation!

    • Con-current nucleophilic substitution AND elimination reactions are discussed at the end of the section on the elimination mechanism where things get even more complicated.

    • Some points concerning the STEREOCHEMISTRY of the reaction :-

    • What happens if the original haloalkane has chirality?

    • AND what is the optical activity of the product?

      • If the haloalkane has three different R groups on the carbon of the C-Hal bond, i.e. RR'R''CX, then there are four different groups bonded to the carbon of the C-Hal bond. Therefore the molecule is chiral and can exhibit optical isomerism (non-super imposable mirror image forms). If the initial halogenoalkane is an optical isomer, the stereochemical consequences depend on which mechanism by which the halogenoalkane reacts. The results can be very complex and their full explanation goes beyond this level. However there are two product formation trends.

        • organic reaction mechanisms

        • (Apologies for repeating diagrams but it helps to appreciate these stereochemical points)

        • In the SN1 carbocation mechanism (e.g. mechanism 1 above), the three bonds of the R groups of the carbocation formed in step (1), are in a trigonal planar arrangement >C-. This means the nucleophile (e.g. OH- or H2O) can attack the carbocation with equal probability on each side. This results in a tendency for a racemic mixture to form, that is an optically inactive mixture of equal amounts of the two optical isomers.

          • (What you actually get in practice is a significant reduction in optical activity in the product)

        • For an optically active halogenoalkane reactant, the considerable reduction in optical activity in a nucleophilic substitution reaction is evidence of the SN1 unimolecular mechanism i.e. the formation of a trigonal planar carbocation.

        • organic reaction mechanisms

        • However, in the case of the SN2 mechanism (e.g. mechanism 33 above), racemisation does NOT take place and chirality and optical activity is completely preserved in the molecule, BUT inversion takes place i.e. the absolute 3D configuration of the product is completely opposite to that of the reactant. Stereochemically the most successful line of attack for SN2 substitution, is if the nucleophile hits the carbon of the C-Hal bond on the opposite side to the halogen atom. The result has been likened to an umbrella being blown inside out in a gale! The three single bonds for the -CRR'R'' are pushed through and so the configuration inverted! [see mechanism 33 style (c)]

        • For an optically active halogenoalkane reactant, the retention of complete optical activity in a nucleophilic substitution reaction is evidence of the SN2 bimolecular mechanism.

    • Reaction progress profiles to show the progress of SN1 and SN2 nucleophilic substitutions.

    (c) doc b SN1 with OH-

(c) doc bSN1 with H2O

  • Reaction profiles above:

  • Diagram 45 corresponds to mechanism 1 and diagram 42 corresponds to mechanism 10.

  • The 'progress' of an SN1 reaction for e.g. tertiary halogenoalkane hydrolysis with sodium hydroxide or water, can be represented by a 'double/triple hump' diagram where the troughs represents the formation of the intermediates. The 2nd activation energy, Ea2, is much smaller than the 1st activation energy, Ea1, because, unlike the 1st step, no bond is broken in the 2nd step and the water/hydroxide ion readily bonds to the carbocation.  Ea3 is also small for the water hydrolysis because protons are readily transferred in acid-base reactions.

organic reaction mechanisms

  • Reaction profile - diagram 41 above (corresponds to mechanism diagrams 2/33/34) The 'progress' of an SN2 reaction e.g. for primary halogenoalkane hydrolysis with sodium hydroxide, can be represented by a 'single hump' diagram where the peak represents the formation of 'activated complex' or 'transition state', in which the 'outgoing' Cl and the 'incoming' OH are 'half-bonded' with the functional group carbon atom.

 


10.4.3 The nucleophilic substitution of halogenoalkane by cyanide ion now on separate page

10.4.4 Nucleophilic substitution of a halogenoalkane with ammonia or primary aliphatic amine now on separate page

10.4.5 The elimination of hydrogen bromide from a bromoalkane now on separate page


keywords phrases: reaction conditions formula intermediates organic chemistry reaction mechanisms nucleophilic substitution R3C-X + OH- ==> R3C-OH + X- R3C-X + 2H2O ==> R3C-OH + X- + H3O+ C6H5CH2Cl + NaOH ==> C6H5CH2OH + NaCl ClC6H4CH3 + NaOH ==> HOC6H4CH3 + NaCl (CH3)3CBr > (CH3)2CHBr > CH3CH2Br > CH3Br (CH3)3C+ > (CH3)2CH+ > CH3CH2+ > CH3+


APPENDIX -  COMPLETE MECHANISM and Organic Synthesis INDEX (so far!)


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