Alkenes and Alkynes 10/27/2010. Chapter 7. Alkenes and Alkynes. Alkenes and Alkynes

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1 Chapter 7 Alkenes and Alkynes CHP6 Problems: , 16-34, 36. CHP7 Problems: , 25-28, 31-34, 37-39, 41-47, Alkenes and Alkynes Alkene (or olefin ) Hydrocarbon that contains a carbon-carbon double bond Present in most organic and biological molecules β-carotene Alkyne Hydrocarbon that contains a carbon-carbon triple bond Rarely occur in biological molecules or pathways Alkenes and Alkynes Ethylene and propylene are the two most important industrially produced organic chemicals Produced by cracking C 2 -C 8 alkanes upon heating to temperatures up to 900 o C. 1

2 7.1 Calculating a Degree of Unsaturation Because of its double bond an alkene has fewer hydrogens than an alkane with the same number of carbons and is therefore referred to as unsaturated Alkene Alkane C n H 2n C n H 2n+2 C 2 H 4 C 2 H 6 Calculating a Degree of Unsaturation Degree of unsaturation Number of rings and/or multiple bonds present in the molecule Unknown hydrocarbon of formula C 6 H 10 with molecular weight of 82 has two fewer pairs of hydrogens (H 14 H 10 = H 4 = 2H 2 ) than a saturated hydrocarbon Degree of unsaturation is two Possible structures for unknown Calculating a Degree of Unsaturation Calculations for compounds containing other elements in addition to carbon and hydrogen: Organohalogen compounds (C,H,X, where X = F, Cl, Br, or I) Add number of halogens and hydrogens to arrive at an equivalent hydrocarbon formula 2

3 Calculating a Degree of Unsaturation Organooxygen compounds (C,H,O) Oxygen does not affect the formula of an equivalent hydrocarbon Ignore the number of oxygens Calculating a Degree of Unsaturation Organonitrogen compounds (C,H,N) Has one more hydrogen than a related hydrocarbon Subtract the number of nitrogens from the number of hydrogens for equivalent hydrocarbon formula 7.2 Naming Alkenes Similar to the naming rules for alkanes Step 1 - Name the parent hydrocarbon Find longest carbon chain containing the double bond Step 2 Number the carbon atoms in the chain Double-bond carbons should receive lowest possible numbers Begin at end nearer first branch point if double bond is equidistant from the two ends 3

4 Step 3 - Write the full name using suffix -ene in place of - ane If more than one double bond indicate position and use suffixes -diene, -triene, and so on Older naming system still in use which places the locant, or number locating the position of the double bond, at the beginning of the name Cycloalkenes Number cycloalkene so double bond is between C1 and C2 and first substituent has lowest number possible 4

5 Common names that are often used and are recognized by IUPAC: Alkynes Named just like alkenes using suffix -yne Number main chain so triple bond receives as low a number as possible Alkyl, alkenyl, and alkynyl groups: 7.3 Cis-Trans Isomerism in Alkenes Carbon-carbon double bond description Valence bond language Carbons are sp2 hybridized Three equivalent hybrid orbitals that lie in a plane at angles of 120º to one another Carbons form a σ bond by head-on overlap of sp2 orbitals and a p bond by sideways overlap of unhybridized p orbitals oriented perpendicular to sp2 plane 5

6 Cis-Trans Isomerism in Alkenes Molecular orbital language Interaction between p orbitals leads to one bonding and one antibonding molecular orbital bonding MO contains no node between nuclei antibonding MO contains a node between nuclei resulting from combination of lobes with different algebraic signs Cis-Trans Isomerism in Alkenes Free rotation is not possible around a double bond The barrier to double bond rotation must be at least as great as the strength of the bond itself (~ 350 kj/mol) Cis-Trans Isomerism in Alkenes Disubstituted alkene Two substituents other than hydrogen are attached to the double-bond carbons But-2-ene Two isomers cannot interconvert spontaneously Methyl groups are cis- or trans- to each other Bond rotation cannot occur the two but-2-enes are cis-trans stereoisomers 6

7 Cis-Trans Isomerism in Alkenes Cis-trans isomerism occurs when both double-bond carbons are bonded to two different groups Carbons bonded to two identical groups cannot exist as cistrans isomers 7.4 Alkene Stereochemistry and the E,Z Designation E,Z system Sequence rules used to assign priorities to the substituent groups on the double-bond carbons (alkenes) E double bond For German entgegen meaning opposite Higher priority groups on each carbon are on opposite sides of double-bond Z double bond For German zusammen meaning together Higher priority groups on each carbon are on same side of doublebond Alkene Stereochemistry and the E,Z Designation Cahn-Ingold-Prelog sequence rules Rule 1 look at the two atoms directly attached to each double-bond carbon and rank them according to atomic number 7

8 Alkene Stereochemistry and the E,Z Designation Rule 2 If a decision cannot be reached by ranking the first atom in the substituent, look at the second, third, and fourth atoms away from the double-bond carbons until the first difference is found Alkene Stereochemistry and the E,Z Designation Rule 3 multiple bonded atoms are equivalent to the same number of single-bonded atoms Worked Example 7.1 Assign E and Z Configurations to Substituted Alkenes Assign E or Z Configuration to the double bond in the following compound: 8