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Topic 2b

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Western University
Chemistry 2223B
Felix Lee

Chemistry 2213a  Fall 2012  Western University Topic 2b. Organic Structure #2: Alkenes & Alkynes A. Structure • Alkenes with one C=C bond have the formula C H , nh2ne alkynes with one C≡C bond have the formula C H . Such compounds are said to be n 2n-2 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 (an index of hydrogen deficiency of one). or –2H –2H or or C H 5 12 C5H 10 C5 8 saturate 1 unit of unsaturation 2 unis of unatuation • So, each unit of unsaturation can be a π bond or a ring. Organic Structure #2: Alkenes & Alkynes  2 • When there are two or more multiple bonds, they can arranged as follows: 1.Isolated: two or more σ bonds between π bonds 2.Conjugated: multiple bonds alternate with a σ bond 3.Cumulated: two double bonds on a single carbon. C iolated conjugated cum ulated 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. 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 Organic Structure #2: Alkenes & Alkynes  3 • Structurally, an alkene consists of two adjacent sp -hybridized atoms (trigonal 2 planar). The three sprbitals are used to form σ bonds. The remaining p orbital overlaps sideways with the p from the adjacent atom to form one π bond. π bonds have electron density above and below the molecular plane. H H C C H H • An alkyne consists of two adjacent sp-hybridized atoms (linear). alkenes, the hybrid orbitals are for σ bonds. The two π bonds originate from the sideways overlap of two p orbitals from each of the atoms. H C C H Organic Structure #2: Alkenes & Alkynes  4 B. cis-trans Isomerism of Alkenes • A multiple bond prevents free rotation about the involved atoms. Such rotation would require breaking the π bond and is not energetically favourable.As a result, cis-trans (geometric) isomers can exist for non-symmetric alkenes. H H H CH CH 2 3 C C C C H3C CH 2H 3 H3C H cis-2-pentene trans-2-pentene mp = –179°, bp = 38° mp = –136°, bp = 36° • In this context, the terms cis-trans refer to the relative orientation of designated substituents on the C=C bond. • cis-trans also used to describe the orientations of substituents bonded to a cycloalkane ring. Do not confuse the two different uses. Organic Structure #2: Alkenes & Alkynes  5 • Recall that if any one of the two carbons has two identical groups, there cannot be cis-trans isomerism. The molecule shown below has no stereoisomers. H C H H 3 Cl 3 C C same as C C H C C l H C H 3 3 • Also, in ring compounds of 7 carbons or less, the double bond must be cis(as defined by the carbon atoms). cis trans cis • For every C=C in a molecule, there is a possibility of 2 cis-trans isomers. So, n for n C=C’s in a molecule, there is a maximum of 2ssible isomers. • The actual number of isomers is decreased if one or more of the double bonds is symmetrical (i.e. has two identical substituents on one of the C=C atoms). o CH CH=CHCH=CHCH=CH has three C=C, but the o
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