alkanes structure and naming (c) doc b(c) doc bDoc Brown's Advanced A Level Chemistry

Revising Advanced Level Organic Chemistry

PART 13 The Shapes of Organic Molecules and bond angles related to their Electronic Structure

Physical Organic Chemistry

or Part 3 of shapes of organic molecules and bond angles

A description, explanation, shapes and bond angles of a variety of organic molecules are described with dot and cross and '3D' shape diagrams to represent e.g. alkanes, halogenoalkanes, alkenes, alkynes, alcohols, carboxylic acids and derivatives, arenes, phenols etc. All is described and explained!

SHAPES OF MOLECULES INDEX: Introduction - theory - lots of inorganic molecule/ion examples * Some other molecules/ions of carbon, nitrogen, sulphur and chlorine * GCSE/IGCSE/A Level Science-CHEMISTRY bonding notes

Organic molecules - two starter examples

These often are not given a particular shape name, but never-the-less, an appreciation of the 3D spatial arrangement is expected e.g.

Methane has the perfect tetrahedral angle of 109.5o (details)

Ethane consists of two joined 'pyramidal halves', in which all C-C-H and H-C-H tetrahedral bond angles are ~109o.

The quoted H-C-C bond angle is 111o and H-C-H bond angle 107.4o

The bulky methyl group reduces the H-C-H angle, but increases the H-C-C bond angle.

alkanes structure and naming (c) doc balkanes structure and naming (c) doc b

ball and stick model of ethane

space-filling model

(c) doc b or (c) doc b

Benzene is a completely planar molecule, with predicted C-C-C or C-C-H bond angles of ~120o.


More examples of the shapes and bond angles of organic molecules

Most bond angles in organic chemistry can be accurately or approximately predicted using bond repulsion theory (with some notable exceptions at the end).

Two groups of bonding electrons around the 'middle' atom of the bond give an angle of 180o.

Alkynes have a single bond pair and a triple bond pair around the middle carbon.

e.g. the R-C-C angle in R-C(c) doc bC-R alkynes, ethyne H-C(c) doc bC-H has linear shape

Dienes with adjacent double bonds have two double bond pairs around the 'middle' atom.

e.g. the C-C-C angle in >C=C=C<

e.g. in propa-1,2-diene CH2=C=CH2, and buta-1,2-diene CH2=C=CH-CH3

Diazonium salts e.g. the C-N(c) doc bN: bond in diazonium cations like [C6H5-N(c) doc bN:]+ 

Three groups of electrons around the 'middle' atom of the bond give an angle of 120o.

Two single bond pairs and double bond pairs.

The H-C-H, C-C-H angles associated with the alkene group >C=C< which leads to a planar shape for ethene itself.

The angles associated with the carbonyl group >C=O e.g. (i) H-C=O, C-C=O in aldehydes, ketones and carboxylic acids and derivatives and (ii) C-C(=O)-O, C-C(=O)-C  in carboxylic acids and their derivatives.

(i) RCHO, RCOR and (ii) RCOOH, RCOOR, RCOCl, RCONH2 etc.

Two 'averaged 1.5' bonds of the delocalised benzene ring and a single bond pair of the bond attaching an atom to the ring.

e.g. in benzene itself, all the -C-C-C- or -C-C-H bonds are 120o and is a completely planar molecule.

and in benzene derivatives C-C-X where X might be Cl, N (from NO2 or NH2), O (from phenol -OH or ether -OR) etc.

A single bond pair, double bond pairs and a lone pair (e.g. on the N atom, x-ref ammonia).

In diazo compounds R-N=N-R, the R-N=N bond angle is 1200.

Four groups of electrons around the 'middle' atom of the bond usually gives an angle of about 109o. The orbitals would point to the corners of a tetrahedron.

Four single bond pairs give all the C-C-C or H-C-H or C-C-H angles in most (see below) saturated systems e.g. alkanes, chloroalkanes etc..

Also the R-N-R angle in the quaternary ammonium salt ion [NR4]+.

Three single bond pairs and a lone pair (x-ref ammonia above).

e.g. the H-N-H, C-N-H bond angles in amines R-NH2, R-NH-R, :NR3 etc.

Two single bond pairs and two non-bonding lone pairs (x-ref water above).

e.g. the C-O-H angle in alcohols and phenols and the >C-O-C< angle in ethers and esters

Some significant exceptions to the above general rules.

Despite the four single bond pairs, the C-C-C bond angle in cyclopropane is a 'forced' 60o. and the H-C-H angle is over 1090, from a distorted 'tetrahedral' situation. The H-C-H bond angle is 118o.

Despite the four single bond pairs, the C-C-C bond angle in cyclobutane is on face value a forced 90o and the H-C-H angles over 1090. However there is evidence to suggest it oscillates between two bent conformers with an 'average' bond angle of ~109o. Also, half of the 'square' C-C-C is bent at an angle of 28o to the other C-C-C half, so it isn't actually a planar molecule anyway!

For cyclopentane onwards, the ring is 'puckered' with C-C-C, C-C-H and H-C-H bond angles of ~109o.

EXAMPLES analysed and summarised for you ...

but I suggest you sketch some out and mark on all the angles, see the end 'scribbles'

methanol alcohols and ether structure and naming (c) doc ball VSEPR H-C-H, C-O-H and H-C-O angles predicted ~109o

However, as with water, you might expect some reduction in the C-O-H bond angle due to the two lone pairs on the oxygen i.e. lone pair - lone pair repulsion > bond pair - bond pair repulsion. I'm sure this repulsion takes place BUT the more bulky methyl group (compared to hydrogen in water) produces its own increase in repulsion and the effect partly cancels this out giving an experimental C-O-H bond angle of 108.5-109o (~109.5), just a small reduction compared to water. I've seen 104.5o quoted on the internet and its wrong!

The H-C-H and H-C-O bond angles are close to 109o. and fit in with simple VSEPR predictions.

chloromethane (c) doc ball VSEPR  H-C-H and H-C-Cl angles ~109o tetrahedral shape

H-C-H is 110.5o, H-C-Cl ? but will be close to 109o.

propane(c) doc b all VSEPR  H-C-H, C-C-H or C-C-C angles are predicted to be 109o 

ethene(c) doc b(c) doc b,

The VSEPR argument gives H-C-H or H-C=C angles of ~120o in the completely planar molecule of ethene

the actual bond angles are: H-C-H 116.6o and C=C-H 121.7o

The double bond involves delocalisation of one of the two pairs of bonding electrons (σ sigma and π pi bonds)

propene(c) doc b H-C-H 109o in the CH3- group, but the

H-C=C, C-C=C, C=C-H and =CH2 angles are 120o

propyne(c) doc b the H-C(c) doc bC and C(c) doc bC-C angles are 180o giving a linear bond arrangement.

and the (c) doc bC-C-H and H-C-H in -CH3 are 109o so propyne can't have a linear shape

methylbenzene(c) doc b C-C-C in ring 120o and C-C-C(H3) of the planar ring 120o

but the C-C-H of the C-CH3 and the H-C-H in the -CH3 off the ring are 109o 

bromoethane(c) doc ball VSEPR H-C-H, H-C-C, C-C-Br, H-C-Br bond angles are ~109o 

ethanol(c) doc ball VSEPR H-C-H, H-C-C, C-C-O, H-C-O, C-O-H predicted bond angles are ~109o 

The C-O-H might be reduced due to the lone pair - lone pair repulsion on the oxygen atom - see methanol above.

methoxymethane(c) doc ball VSEPR H-C-H, H-C-O, C-O-C angles are predicted ~109o 

However, as with water, you might expect some reduction in the C-O-C bond angle due to the two lone pairs on the oxygen i.e. lone pair - lone pair repulsion > bond pair - bond pair repulsion. I'm sure this repulsion takes place BUT the more bulky methyl groups (compared to hydrogens in water) produce there own increase in repulsion and they cancel out the oxygen lone pair effect giving an experimental C-O-C bond angle of 110o (~109.5), but by coincidence as a result of the two factors, VSEPR correct by default! I've seen 104.5o quoted for the C-O-C bond angle in methoxymethane on the internet based on a water analogy, BUT this is wrong.

phenol(c) doc bC-C-C in ring 120, C-C-H of ring 120o and C-O-H off ring is ~109o 

methylamine (c) doc b expect all H-C-H, C-N-H and H-N-H angles to be ~109o 

However since lone pair - bond pair repulsion > bond pair - bond pair repulsion, you might expect a reduction in the H-N-H and C-N-H bond angles. However the more 'electronically' bulky methyl group overrides this effect and actually produces an increase in one of the bond angles. Quoted values are C-N-H bond angle of 110.3o (but close to 109.5o). BUT the effect of the nitrogen lone pair and the methyl group do result in a H-N-H bond angle reduction to 107.1o, and this you would predict from ammonia (H-N-H bond angle of 107.5o).

ethylamine(c) doc b expect all H-C-H, C-C-H, C-C-N and C-N-H angles are all 109o 

butanone (c) doc b H-C-C, H-C-H and (O=)C-C-C on right are all ~109o 

and C-C=O on left, C-C(=O)-C in the middle, and O=C-C on right are ~120o 

methanoic acid(c) doc bH-C=O, H-C-O(-H) and O=C-O are ~120o and C-O-H is ~109o 

ethanamide (c) doc b  H-C-H, H-C-C, H-N-H and C-N-H are all ~109o 

and C-C=O, C-C-N and O=C-N are ~120o 

ethyl ethanoate (c) doc b  all H-C-H, O-C-C (right), H-C-C (left)

and C-C-H (right) are all _109o, and C-C=O, C-C-O (left) and O=C-O are ~120o 

ethanoyl chloride (c) doc b  H-C-H and H-C-C are ~109o 

and C-C=O, C-C-Cl and O=C-Cl are all ~120o 

diazo dye (c) doc b all the C-C-C, C-C-H, C-N=N, C-C-N(=) bond angles

of/off the ring are all ~120o but the C-O-H of the phenol group on the right is ~109o.

(c) doc b

the SCRIBBLES of some of the above molecules!

which will eventually be replaced by neater diagrams!

SEE ALSO Appendix 1-4 on separate page: The shapes, with ox diagrams and bond angles, of some other molecules/ions of carbon, nitrogen, sulphur and chlorine besides those on this page and again the 'scribbles' will be replaced by neat diagrams eventually!

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