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Advanced A/AS Level Chemistry: Shapes of organic molecules
Doc 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
- 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.
ball and stick model of ethane
Note that around each carbon atom is a tetrahedral arrangement of single bonds (C-H or C-C).
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).
Some significant exceptions to the above general rules.
EXAMPLES analysed and summarised for you ...
methanol all 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 all 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 all VSEPR H-C-H, C-C-H or C-C-C angles are predicted to be 109o
Again, note that around each carbon atom is a tetrahedral arrangement of single bonds (C-H or C-C).
ethene, , ,
propene H-C-H 109o in the CH3- group, but the
propyne the H-CC and CC-C angles are 180o giving a linear bond arrangement.
methylbenzene 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
bromoethaneall VSEPR H-C-H, H-C-C, C-C-Br, H-C-Br bond angles are ~109o
ethanolall 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.
methoxymethaneall 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.
phenolC-C-C in ring 120, C-C-H of ring 120o and C-O-H off ring is ~109o
methylamine 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) 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 109o
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 ~109o
methanoic acidH-C=O, H-C-O(-H) and O=C-O are ~120o and C-O-H is ~109o
ethanamide H-C-H, H-C-C, H-N-H and C-N-H are all ~109o
ethyl ethanoate all H-C-H, O-C-C (right), H-C-C (left)
ethanoyl chloride H-C-H and H-C-C are ~109o
diazo dye all the C-C-C, C-C-H, C-N=N, C-C-N(=) bond angles
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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