Topic 5D

Alkenes and alkynes

5D Alkenes and alkynes

• A sigma () bond between two sp2 hybridised carbons

• A pi () bond formed by sideways overlap of two unhybridized pz orbitals

42Structure of double bond :

Pi bonding

• Pibonds weaker than sigma bonds

• Alkenes are more reactive than alkanes.

• Electrons in p-orbitals further from the nucleus than s-electrons and are less tightly bound

• Reactivity of alkenes involves the -bond, not the -bonds.

43

Bond energy: Total — Sigma = Pi 681 397 284 kJmol–1

Structural consequences of -bonding

43

Rotation means loss of overlap

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Movie from Saunders General Chemistry CD-ROM

Structural consequences of -bonding

• Whereas in alkanesthere is free rotationabout C—C bonds, Alkenes are rigid

• For rotation the -bond must be broken requiring 285 kJmol–1

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Rotation in C—C bonds

• Staggered and eclipsed conformations possible

• Eclipsed higher in energy by 11 kJmol–1

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H

C

H H

C

H H

H

H

C

H H

C

H

H HC–C single bond

rotation about

"eclipsed"conformation

"staggered"conformation

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Restricted rotation in C=C bonds

• Alkenes are however rigid and can have two configurations

• Groups on the double bond may be cis or transeg 2-butene

44

C C

CH3

H

CH3

H

C C

CH3

H

H

CH3transcis

and

2-Butene

Geometrical isomers

• These configurations require different groups on each end of the double bond

• Different forms are called geometrical isomers

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A B

C D

B A

C D

A B

C C

B A

C C

Top of p-orbitalviewed from above

Two faces

Geometrical isomersEquivalent isomers

Shapes of alkynes• Because each carbon is sp hybridised (hybrid

orbitals 180° apart) , ethyne is a linear molecule.

• Pi bonds form a barrel of electron density around the CC bond.

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Naming alkenes

• Suffix "ene" is used to denote alkenes.• Number from end nearest double bond

(giving first carbon of alkene the lowest number)• The stem is based on the longest chain containing the double

bond• Geometrical isomers are identified using the

E–Z nomeclature system

46

Naming alkenes 46

Examples:

CH3CH2CH CH2

12341-Butene

C CHCH 3

CH3CH2CH2

CH3CH2CH2CH2123

4567

3-propyl-2-heptene

12342-ButeneCH3CH CHCH3

Naming alkynes

• Suffix "yne" is used to denote alkynes.• Number from end nearest triple bond

(giving first carbon of alkyne the lowest number)

• The stem is based on the longest chain containing the triple bond

• Linear — no geometrical isomerism

46

C C

Naming alkynes

• ethyne H–CC–H

• propyne CH3–CC–H

• 1-butyne CH3–CH2–CC–H

• 2-butyne CH3–CC–CH3

46

Examples:

The E–Z system

• Distinguishes cis and trans geometrical isomers• A group on each end is given preference using

rules• If these are on the same side — cis — we have

the Z-form. "Z" stands for "zusamen"• If these are on opposite side — trans— we have

the E-form. "E" stands for "entgegen"

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C C

CH3

H

CH3

H

C C

CH3

H

H

CH3transcis

and

The E–Z system 47

Must have different substituents at each endThere is only one form of 2-methyl-2-butene

CH3 H

CH3 CH3

H CH3

CH3 CH3

2-methyl-2-butene

Prioritising groups 48

1. Priorities based on atomic NUMBER of attached atoms

2. If same atoms are attached compare the attached atoms next along the chain

3. Double and triple bonds are treated as follows:

CH2 CH2 CO

CH3Cl C CH

CH2 CH2 ClC C

CO

CH3

CO C CH

C

C

C

C

E/Z designation 48

CH3 H

CH3 H

Z-2-butene (cis)

CH3 H

H CH3

E-2-butene (trans)

• E—priority groups on the opposite face

• Groups assigned priority based on atomic number of attached atom — CH3 has priority over H

• Z—priority groups on the same face

• Oxygen and carbon have priority over hydrogen

• Chlorine and carbon have priority over hydrogen

The E–Z system 48

CH3CH2 H

Cl H

CH3CH2 H

H Cl

1-chloro-1-butene

Cis(Z) Trans(E)

HO H

H CH3

H OH

H CH3

1-propenol

Cis(Z)Trans(E)

• HO has priority over methyl and carbon has priority over hydrogen

• Chlorine and ethyl have priority over hydrogen and methyl

The E–Z system 48

CH3CH2 CH3

Cl H

CH3CH2 CH3

H Cl

1-chloro-2-methyl-1-butene

(Z) (E)

HO CH3

H CH3

CH3 OH

H CH3

2-buten-2-ol

(Z)(E)

The E–Z system 48

• The double bond has priority over methyl

• Methyl has priority over hydrogen

• Priority groups are on the same side

• (Z)-alkene

H3C CH CH2

H CH3

CH2

C

CH

C

(Z)-3-Methylpentadiene

Cycloalkenes

• Alkene carbon is numbered "one"• Number along double bond around ring to the

nearest substituent

49

CH3

1

2

34

5

3-methylcyclopentene

CH3

1

23

45

NOT

would be 5-methylcyclopentene

Seeing is believing 50

Movies from Saunders General Chemistry CD-ROM

Note the isomerisation is actually from cis to trans