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 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/AS 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.

alkanes structure and naming (c) doc b

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

ball and stick model of ethane

space-filling model

(c) doc b or (c) doc b

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

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.

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 angle is over 1090. However there is evidence to suggest it oscillates between two bent conformers with an 'average' bond angle of ~109o.

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 H-C-H, C-O-H and H-C-O angles ~109o

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

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

ethene(c) doc b (c) doc b

all H-C-H or H-C=C angles are 120o in the completely planar molecule of ethene

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

and the (c) doc bC-C-H and H-C-H in -CH3 are 109o linear molecule shape

methylbenzene(c) doc b C-C-C in ring 120o and C-C-C(H3) off the 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 H-C-H, H-C-C, C-C-Br, H-C-Br angles are 109o 

ethanol(c) doc ball H-C-H, H-C-C, C-C-O, H-C-O, C-O-H angles are 109o 

methoxymethane(c) doc ball H-C-H, H-C-O, C-O-C angles are all 109o 

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

ethylamine(c) doc b 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 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.

the SCRIBBLES!(c) doc b

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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