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Chapter 7

Alkenes and Alkynes

• CHP6 Problems: 6.1-13, 16-34, 36.

• CHP7 Problems: 7.1-23, 25-28, 31-34, 37-39, 41-47,

49-56.

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” C2-C8

alkanes upon heating

to temperatures up to

900 oC.

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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

CnH2n CnH2n+2

C2H4 C2H6

7.1 Calculating a Degree of Unsaturation

Degree of unsaturation

• Number of rings and/or multiple bonds present in the

molecule

• Unknown hydrocarbon of formula C6H10with molecular weight

of 82 has two fewer pairs of hydrogens (H14 – H10 = H4 = 2H2)

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

Calculating a Degree of Unsaturation

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• 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

Calculating a Degree of Unsaturation

7.2 Naming Alkenes and Alkynes

• 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

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• 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

Naming Alkenes and Alkynes

• 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

Naming Alkenes and Alkynes

• Cycloalkenes

• Number cycloalkene so double bond is between C1

and C2 and first substituent has lowest number

possible

Naming Alkenes and Alkynes

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Common names that are often used and are recognized

by IUPAC:

Naming Alkenes and Alkynes

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:

Naming Alkenes and Alkynes

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

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• 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

Cis-Trans Isomerism in Alkenes

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Cis-trans isomerism occurs when both double-bond

carbons are bonded to two different groups

• Carbons bonded to two identical groups cannot exist as cis-

trans isomers

Cis-Trans Isomerism in Alkenes

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 double-bond

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

Alkene Stereochemistry and the E,Z Designation

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• 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

Alkene Stereochemistry and the E,Z Designation

Assign E or Z Configuration to the double bond in the

following compound:

Worked Example 7.1

Assign E and Z Configurations to Substituted

Alkenes