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 180^o.

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

e.g. the R-C-C angle in R-CC-R alkynes, ethyne H-CC-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-NN: bond in diazonium cations like [C₆H₅-NN:]⁺ 

 

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

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

(ii) C-C(=O)-O, C-C(=O)-C  in carboxylic acids and their derivatives.

(iii) 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 ~120^o 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 109^o. 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' 60^o. and the H-C-H angle is over 109⁰, from a distorted 'tetrahedral' situation. The H-C-H bond angle is 118^o.

Despite the four single bond pairs, the C-C-C bond angle in cyclobutane is on face value a forced 90^o 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 ~109^o. Also, half of the 'square' C-C-C is bent at an angle of 28^o to the other C-C-C half, so it isn't actually a planar molecule anyway!

For cyclopentane onwards, the ring is actually 'puckered' or 'bent' with C-C-C, C-C-H and H-C-H bond angles of ~109^o e.g. cyclohexane shown on the right. The simple hexagonal skeletal formula for cyclohexane is misleading, the C-C-C bond angles are NOT 120^o!

EXAMPLES analysed and summarised for you  below ...

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

methanol

all VSEPR H-C-H, C-O-H and H-C-O angles predicted ~109^o

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 ~108.5-109^o, just a small reduction compared to water.

I've seen 104.5^o quoted for C-O-H bond angle on the internet and its wrong!

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

chloromethane

all VSEPR  H-C-H and H-C-Cl angles ~109^o tetrahedral shape

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

propane  

all VSEPR  H-C-H, C-C-H or C-C-C angles are predicted to be ~109^o 

Again, note that around each carbon atom is a tetrahedral arrangement of single bonds (C-H or C-C).

ethene

,  ,  ,

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

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

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

propene H-C-H 109^o in the CH₃- group, but the

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

propyne

the H-CC and CC-C angles are 180^o giving a linear bond arrangement.

and the C-C-H and H-C-H in -CH₃ are 109^o so propyne can't have a linear shape

methylbenzene

C-C-C in ring 120^o and C-C-C(H₃) of the planar ring 120^o

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

bromoethane

all VSEPR H-C-H, H-C-C, C-C-Br, H-C-Br bond angles are ~109^o 

ethanol

all VSEPR H-C-H, H-C-C, C-C-O, H-C-O, C-O-H predicted bond angles are ~109^o 

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

methoxymethane

all VSEPR H-C-H, H-C-O, C-O-C angles are predicted ~109^o 

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 110^o (~109.5), but by coincidence as a result of the two factors, VSEPR correct by default!

I've seen 104.5^o quoted for the C-O-C bond angle in methoxymethane on the internet based on a water analogy, BUT this is wrong.

phenol

C-C-C in ring 120, C-C-H of ring 120^o and C-O-H off ring is ~109^o 

methylamine

expect all H-C-H, C-N-H and H-N-H angles to be ~109^o 

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 110.3^o (but close to 109.5^o).

BUT, the effect of the nitrogen lone pair and the methyl group, do result in a H-N-H bond angle reduction to 107.1^o, and this you would predict from ammonia (H-N-H bond angle of 107.5^o) due to extra lone pair - bond pair repulsion > bond pair - bond pair repulsion.

ethylamine

expect all H-C-H, C-C-H, C-C-N and C-N-H angles are all 109^o 

I don't know if the H-N-H bond angle is ~107 like ammonia and methylamine.

butanone

H-C-C, H-C-H and (O=)C-C-C on right are all ~109^o 

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

methanoic acid

H-C=O, H-C-O(-H) and O=C-O are ~120^o and C-O-H is ~109^o 

ethanamide  

H-C-H, H-C-C, H-N-H and C-N-H are all ~109^o 

and C-C=O, C-C-N and O=C-N are ~120^o 

ethyl ethanoate  

all H-C-H, O-C-C (right), H-C-C (left)

and C-C-H (right) are all _109^o, and C-C=O, C-C-O (left) and O=C-O are ~120^o 

ethanoyl chloride  

H-C-H and H-C-C are ~109^o 

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

diazo dye

all the C-C-C, C-C-H, C-N=N, C-C-N(=) bond angles

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

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, sulfur and chlorine besides those on this page and again the 'scribbles' will be replaced by neat diagrams eventually!