alkenes and alkynes

Alkenes and Alkynes

Structure and Properties

NomenclatureSynthesis of AlkenesReactions of Alkenes

Alkenes - Synthesis and Reactions


Alkene: hydrocarbon with one or more C-C

double bond also called olefin

C=C consists of 1 bondand 1 bond

Ethylene

ethene

C C

H

H

H

H


C=C is a functional group BDE ( bond) = ~83 kcal/mol BDE ( bond) = ~ 63 kcal/mol

bond is weaker than bondreactions take place at the bond

sp2 hybridization trigonal planar


Trigonal planar geometry approximately 120o bond angle for

alkenesvs. ~109.5o bond angle for alkanes

Double bonds are shorter than single bonds.


Alkanes saturated hydrocarbons

each C has the maximum # of H’s possible

Alkenes unsaturated hydrocarbons

fewer H atoms per C than an alkanecapable of adding hydrogen

CH3CH

3

CH2 CH

2

CH3CH

3

CH2 CH

2

Structure and Properties Element of unsaturation

a structural feature that reduces the number of hydrogen atoms by 2 relative to the corresponding alkanering bond

used to help determine possible structures

CH3CH

3

CH2 CH

2

CH3CH

3

CH2 CH

2

CH3CH

3

CH2 CH

2

CH3CH

3

CH2

C6H

14

CH2

CH3CH

3

CH2

C6H

12

CH2

CH3CH

3

CH2

C6H

12

CH2


Elements of unsaturation = 1/2 (2C + 2 - H)

C6H12

EU = ½ (2x6 +2 – 12) = 1


Example: Calculate the elements of unsaturation for C4H8. Draw 5 structural isomers with this formula.


5 structural isomers of C4H8


To determine the elements of unsaturation for compounds with heteroatoms (atoms other than C and H): use same formula as given previously

BUT

Each halogen counts as a hydrogen atom

Ignore any oxygen atoms Each nitrogen counts as 1/2 C


Example: Calculate the elements of unsaturation for C6H9ClO. Draw at least 4 structural isomers.


4 possible structural isomers

The structures you draw should contain reasonable functional groups….i.e. don’t make up strange functional groups!

IR Alkenes have two characteristic peaks in the

IR: sp2 C-H at >3000 cm-1

C=C at ~1620 – 1680 cm-1

Conjugated alkene C=C is at lower frequency

Isolated alkene C=C is at higher frequency

C=C peak has variable intensity but is typically weak to moderate.

Alkene

C=C

sp2 C-H

sp3

C-H

Nomenclature

Alkenes can be named using either IUPAC names or common names.

CH2=CH

2

CH2=CHCH

3

CH2=C

CH3

CH3

CH2=CH

2

CH2=CHCH

3

CH2=C

CH3

CH3

CH2=CH

2

CH2=CHCH

3

CH2=C

CH3

CH3

etheneethylene

propenepropylene

2-methylpropeneisobutylene

Blue = IUPACRed = common

Nomenclature

To name alkenes: Find the longest continuous chain (or

ring) that contains the double bond.Base name = name of corresponding alkane or cycloalkane with ending changed to “ene”

Cl Hexane hexene

Cl

Br

BrCH

3

cyclopentane cyclopentene

Nomenclature

To name alkenes: Number from the end of the chain

closest to the double bondthe double bond is given the lower number of the two double-bonded carbons

Cycloalkenes: double bond is always between carbons 1 and 2

Cl

Br

BrCH

3

1

2 4

3 5

6 1

23

4

5

Cl

Br

BrCH

3

Nomenclature Place the number of the double bond in

front of the base name of the alkene (omit the number for cycloalkenes unless > 2 double bonds)

Cl

Br

BrCH

3

a substituted 2-hexene

a substituted cyclopentene

a substituted hex-2-ene Newer IUPAC system places the

position number just before the “ene” ending

Cl

Br

BrCH

3

Nomenclature

Name substitutent groups in the same manner as in alkanes.

Cl

Br

BrCH

3

trans-6-chloro-5-methyl-2-hexeneor

trans-6-chloro-5-methylhex-2-ene

3-bromo-4-methylcyclopentene

Cl

Br

BrCH

3

Nomenclature

Alkenes as substitutents (often named using common names)

CH2

CH2

CH CH2

CH2

CH2

CH CH2

CH2CH

CHCH2Cl

Methylene group

vinyl group

Allyl group

3-methylenecyclohexene

3-vinyl-1,5-hexadiene3-vinylhexa-1,5-diene

Allyl chloride

Nomenclature

For compounds that show geometric isomerism, add the appropriate prefix: cis trans

OR E Z

NOTE: Cycloalkenes are assumed to be cis unless otherwise indicated.

Nomenclature

Cis/trans isomers

C CA

AB

CC C

A

C

A

B

cis trans

Cis: 2 identical groups located on the same side of the double bond

Trans: 2 identical groups located on opposite sides of the double bond

Nomenclature

Example: Name the following compounds.

Br

CH3

Nomenclature

Some compounds form geometric isomers that cannot be named using the cis/trans nomenclature

Cis/trans nomenclature can’t be used: two identical groups are not attached

to adjacent carbons in the C=C

C

C

CCH3

C

Br

Cl H

Cl

Br

CH3

H

C

C

CCH3

C

Br

Cl H

Cl

Br

CH3

H

Nomenclature

The E-Z system of nomenclature for geometric isomers: Break the double bond into two halves

Separately, assign priorities to the groups on each carbon in the double bond using the Cahn-Ingold-Prelog rules (R & S configuration rules)

C

C

CCH3

C

Br

Cl H

Cl

Br

CH3

H

C C

C

C

CCH3

C

Br

Cl H

Cl

Br

CH3

H

C C

C

C

CCH3

C

Br

Cl H

Cl

Br

CH3

H

C C

1

2

1

2

Nomenclature Z (Zusammen) isomer

both high priority groups are on the same side of the double bondsimilar to cis

E (Entgegen) isomer high priority groups are on the

opposite side of the double bondsimilar to trans

C

C

CCH

3

C

Br

Cl H

C C

1 1

22 C

C

CCH

3

C

Br

Cl H

C C

1

2 1

2

CH2=CH

2

CH2=CHCH

3

CH2=C

CH3

CH3

C CBr

Cl

CH3

H (Z)-1-bromo-1-chloropropene

Nomenclature

Naming alkenes with more than one double bond: Make sure that the longest chain

includes as many C=C as possible.2 C=C diene3 C=C triene4 C=C tetraene

Br

a substituted octatriene

Nomenclature

Show the location of each double bond

Designate the isomer present for each double bond (use location and E or Z)

Br

3-bromo-2, 4, 6-octatriene3-bromoocta-2,4,6-triene

Br(2Z,4E,6E)-3-bromo-2,4,6-

octatriene(2Z,4E,6E)-3-bromoocta-2,4,6-

triene

Nomenclature

Example: Name the following compounds.

BrBr

Nomenclature

Example: Draw the following compounds.

cis-3-methyl-2-pentene

1-ethylcyclohexene

(2E, 4Z)-2,4-hexadiene

Remember: You must show the trigonal planar geometry around the C=C.

Uses and Physical Properties Alkenes are important intermediates in the

synthesis of polymers, drugs, pesticides, and other chemicals.

Ethylene is used as a feedstock for: ethanol ethylene glycol (antifreeze) acetic acid

Propylene is used as a feedstock for: isopropyl alcohol acetone

Uses and Physical Properties Alkenes are important “monomers” for

the production of polymers like poly(vinyl chloride), and Teflon.

Uses and Physical Properties

Physical Properties Similar to alkanes Density

~0.6 g/mL to ~ 0.7 g/mL Boiling Point

increases with increasing MWdecreases with branching

Polarityrelatively non-polar

insoluble in water

Stability of Alkenes

The heat of hydrogenation is used to compare the relative stabilities of alkenes. Heat of hydrogenation:

The heat released (H) during a catalytic hydrogenation

Catalytic hydrogenation: the addition of H2 to a double (or triple) bond in the presence of a catalyst

C

H3C

CH3CH=CHCH3 + H2 CH3CH2CH2CH3

CH3C CH3

HH

Pt


As the heat of hydrogenation becomes more negative, the stability of the alkene decreases.


More highly substituted double bonds are more stable larger angular separation between the

bulky alkyl groups


For acyclic alkenes, trans isomers are more stable than cis isomers.

Trans isomers of cycloalkenes with fewer than 8 carbons are unstable. Large amount of ring strain

Because of ring strain, cycloalkenes with less than 5 carbons in the ring are less stable than those with 5 or more carbons.


Example: Which of the following alkenes is more stable.

vs.

vs.

alkenes and alkynes

Documents

cc double bondalso

conjugated alkene c

double bondthe double

csp2 chsp3c

kcalmolp bond

double bondsa

kcalmolbde p bond

o bond angle