Concept 04 / 08Organic Chemistry - Basic Principles and Techniques
Organic Compounds: Tetravalence of Carbon, Structural Representations and Classification
Tetravalence and hybridisation of carbon in organic molecules, sigma and pi bonds, complete, condensed and bond-line formulas, three-dimensional wedge-dash representation, classification of organic compounds into acyclic, alicyclic, aromatic and heterocyclic types, homologous series, functional groups and degree of unsaturation.
Organic chemistry is the chemistry of carbon compounds (other than simple ones like CO, and carbonates). Carbon's ability to catenate and to form strong single and multiple bonds with itself and with H, O, N, S and halogens gives millions of compounds.
Tetravalence and hybridisation. Carbon forms four covalent bonds.
: four single bonds, tetrahedral (), as in alkanes.
: one double bond, trigonal planar (), as in alkenes and carbonyl compounds.
: a triple bond or two cumulated double bonds, linear (), as in alkynes and nitriles.
More s character means a shorter bond, a more electronegative carbon () and, for C-H, a more acidic hydrogen (ethyne is far more acidic than ethane). A bond forms by sideways overlap of parallel p orbitals; it is weaker than a bond, prevents free rotation, and its electrons are exposed to attack by electrophiles.
Structural representations (see figure).
Complete (expanded) formula: every bond shown by a line.
Condensed formula: bonds omitted, identical groups grouped: , or for hexane.
Bond-line formula: carbons are the corners and line ends of a zigzag; hydrogens on carbon are not drawn; heteroatoms and their hydrogens are shown.
Three-dimensional: wedges for bonds towards the viewer, dashes for bonds away, plain lines in the plane of the paper.
Classification.
Acyclic (open chain, aliphatic): straight or branched chains.
Cyclic: alicyclic (carbocyclic, non-aromatic: cyclohexane), aromatic (benzenoid: benzene, naphthalene; non-benzenoid: tropolone), and heterocyclic (ring containing N, O or S: pyridine, furan, thiophene).
Homologous series: a family with the same functional group, successive members differing by (mass ), with a general formula and gradually changing physical properties (alkanes , alkenes , alcohols ).
Functional group: the atom or group that determines characteristic reactions: , , , , , , , , and so on.
Degree of unsaturation (double bond equivalents). For :
(oxygen does not count).
Each ring or double bond counts ; a triple bond counts ; benzene has DBE .
Key Takeaways:
- , , ; more s character, more electronegative carbon.
- Bond-line formulas omit C and the H on C.
- Homologues differ by ; the functional group decides the chemistry.
- .
- Pitfall: in bond-line formulas, the hydrogens on heteroatoms (OH, ) must be shown.
Brain-Teaser 1: Find the hybridisation of each carbon in and count its and bonds.
Solution: C1 () , C2 (CH) , C3 (nitrile carbon) . Bonds: three C-H, one C-C, one C=C, one . : ; : .
Brain-Teaser 2: Calculate the degree of unsaturation of (nitrobenzene) and of , and suggest structures for the latter.
Solution: Nitrobenzene: (benzene ring plus one N=O). : : a diene (buta-1,3-diene), an alkyne (but-1-yne, but-2-yne), a cycloalkene (cyclobutene), or a bicyclic compound.
Brain-Teaser 3: Why is the C-H bond in ethyne more acidic than that in ethene or ethane?
Solution: The carbon has s character, so it holds the electron pair of the carbanion closer to the nucleus, stabilising . Ethene () and ethane () carbanions are less stable. Ethyne reacts with sodium or sodamide to form sodium acetylide; ethane does not.
Practice this concept
Free · no account required · instant solutions