Pre-university Advanced A Level Chemistry: Shapes of organic molecules and bond angles
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Doc Brown's Advanced A Level Chemistry Revising Advanced Level Organic Chemistry or Physical Chemistry PART 13 The shapes and bond angles of organic molecules related to their electronic structure email doc brown - comments - query? All by structure and chemical bonding revision notes
SHAPES OF MOLECULES INDEX: two other pages
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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. |
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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). space-filling model |
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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 ...
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
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
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! Revision notes for GCE Advanced Subsidiary Level AS Advanced Level A2 IB Revise AQA GCE Chemistry OCR GCE Chemistry Edexcel GCE Chemistry Salters Chemistry CIE Chemistry, WJEC GCE AS A2 Chemistry, CCEA/CEA GCE AS A2 Chemistry revising courses for pre-university students (equal to US grade 11 and grade 12 and AP Honours/honors level courses)
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