4. Alkenes and Alkynes

4 Alkenes and - 2010 Department of Chemistry The University of Western Ontario 4.1 4 Alkenes and Alkynes(text 4.1 4.4 A Structure and Nomenclature

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© 2010, Department of Chemistry, The University of Western Ontario 4.1 4. Alkenes and Alkynes (text 4.1 – 4.4) A. Structure and Nomenclature Alkenes with one C=C bond have the formula C n H 2n . Alkynes with one C C bond have the formula C n H 2n-2 . Such compounds are said to be unsaturated because they have less than the maximum number of hydrogens. Each reduction of two H from the general alkane formula represents one unit of unsaturation (one index of hydrogen deficiency). C 5 H 12 saturated –2H C 5 H 10 1 unit of unsaturation or C 5 H 8 or –2H or 2 units of unsaturation So, each unit of unsaturation can be a bond or a ring. When there are two or more multiple bonds, the arrangements can be as follows: 1. Isolated: two or more bonds between bonds 2. Conjugated: multiple bonds alternate with a bond 3. Cumulated: two double bonds on a single carbon.
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4.2 C isolated conjugated cumulated What can be said about the 2p orbitals of a conjugated system? In systematic nomenclature, the multiple bond (for C=C and C C) is given the lowest possible number on the parent chain. This fixes the chain numbering, and C=C (alkene) has the suffix ene , and C C (alkyne) the suffix yne . 3,5-dimethyl-2-hexene 3- n -butyl-1,4-pentadiyne Many simple compounds are known by common names: o H 2 C=CH 2 ethylene IUPAC = ethene o HC CH acetylene IUPAC = ethyne o CH 3 CH=CH 2 propylene IUPAC = propene Some substituents are also known by their common names: o H 2 C=CH– vinyl H 2 C=CH 2 –Cl vinyl chloride o H 2 C=CHCH 2 – allyl H 2 C=CHCH 2 –Br allyl bromide o HC CCH 2 propargyl propargyl cyclopentane
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4.3 B. Shapes of Alkenes and Alkynes Alkene: each C is bonded to three groups and is sp 2 (planar) with a bond angle of 120°.
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This note was uploaded on 05/01/2011 for the course CHEMISTRY Chemistry taught by Professor Dr.felixlee during the Fall '11 term at UWO.

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4 Alkenes and - 2010 Department of Chemistry The University of Western Ontario 4.1 4 Alkenes and Alkynes(text 4.1 4.4 A Structure and Nomenclature

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