chem 1152: ch. 12 unsaturated hydrocarbons. types of hydrocarbons saturated: all c—c bonds are...

Chem 1152: Ch. 12

Unsaturated Hydrocarbons

Types of HydrocarbonsTypes of Hydrocarbons• Saturated: All C—C bonds are single bonds• Unsaturated: Hydrocarbons with one or more multiple bonds• The term unsaturated is used because C could bind more H (become

saturated) if no multiple bonds existed.

1. Alkenes (ethylene)

C C

H

H H

H

2. Alkynes (acetylene)

C CH H

3. Aromatics (benzene)

H

H

H

H

H

H

Rules for Naming AlkenesRules for Naming Alkenes

1. Name the longest chain that contains the double bond or double bonds. The name of the chain will end in –ene.

2. Number this longest chain so the C=C bond or bonds has/have the lowest number.

3. The first C of the C=C bond (for C=C bond to have lowest number) identifies the positional location of the double bond.

4. Name the attached functional groups.5. Combine the names of the attached groups and longest

chain, the same as you would with alkanes.6. For multiple double bonds, indicate the locations of all

multiple bonds, use numeric prefixes indicating number of double bonds (-diene, -triene).

Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

Naming AlkenesNaming Alkenes• Step 1: Name the longest chain that contains the C=C bond.

Use the IUPAC root and the –ene ending.• Step 2: Number the longest chain so the C=C bond gets the

lowest number possible.• Step 3: Locate the C=C bond with the lower-numbered

carbon. Examples: 1 2 3 4 CH3-CH=CH-CH3

2-butene

6 5 4 3 2 1 CH3-CH2-CH2-CH=CH-CH3

2-hexene

Naming Alkenes (continued)Naming Alkenes (continued)

• Step 4: Locate and name attached groups.• Step 5: Combine all the names.

Naming AlkenesNaming Alkenes


CH3 CH3CH CH CH2

CH3 CH2

C CH2

CH2CH2CH2CH3

C

CH2CH2CH2CH3

CH2 CH2 CH3

CH3

C

CH3 H

2-pentene

2-ethyl-1-hexene

5-ethyl-3-methyl-2-octene

C

CH2CH2

CH2 C

C

CH3

CH2CH2

CH3

Br

CH

4-methylcyclohexene

3-bromo-2-methyl-1-propene

Alkene ExamplesAlkene Examples

2-methylpent-2-ene (3Z)-3-methylhex-3-ene

Br

(3E)-3-bromo-4-methylhex-3-ene

(3Z)-3,4,5-trimethylhept-3-ene

ICl

(2Z,4E)-1-chloro-5-iodo-2,4-dimethylhexa-2,4-diene

(4Z)-3,4,7-trimethylocta-1,4,6-triene

(2E)-but-2-ene

4-methylcyclohexene

Br

(5S)-5-bromocyclohexa-1,3-diene

H

Cl

5-chlorocyclopenta-1,3-diene

The Geometry of AlkenesThe Geometry of Alkenes

• In C=C bonds, sp2 hybrid orbitals are formed by the carbon atoms, with one electron left in a 2p orbital. A representation of sp2 hybridization of carbon.

• During hybridization, two of the 2p orbitals mix with the single 2s orbital to produce three sp2 hybrid orbitals. One 2p orbital is not hybridized and remains unchanged.

2p2s1s

Energy

2p3 sp2

1s

The Geometry of Alkenes (continued)The Geometry of Alkenes (continued)

1. One bond (sigma, σ) is formed by overlap of two sp2 hybrids.

2. The second bond (pi, π) is formed by connecting the unhybridized p orbitals.

2p2s1s

Energy

2p3 sp2

1s



– The planar geometry of the sp2 hybrid orbitals and the ability of the 2p electron to form a “pi bond” bridge locks the C=C bond firmly in place.



• Because there is no free rotation about the C=C bond, geometric isomerism is possible.

• cis- isomers have two similar or identical groups on the same side of the double bond.

• trans- isomers have two similar or identical groups on opposite sides of the double bond.


The Geometry of Alkenes (continued)The Geometry of Alkenes (continued)• Geometric isomers have different physical properties.


Geometric (cis-trans) Isomerism in AlkenesGeometric (cis-trans) Isomerism in Alkenes• By definition, must have 2 different groups attached to each double

bonded carbon.

HH

C C

CH3 CH3

H

C C

CH3 H

CH3

HH

C C

CH3 CH3

CH3

C C

H H

CH3

HH

C C

CH3 H

H

C C

H H

CH3

Geometric Isomers?

Yes

No

No

Which of the following can exhibit cis-trans isomerism? Cl-CH=CH-Cl CH2=CHCH3 Cl-CH=CHCH3

Properties of Alkanes and AlkenesProperties of Alkanes and Alkenes


Alkanes Alkenes Non-polar Water insoluble Alkanes have lower MP and

lower BP than other organic compounds

Odorless Less dense than water As alkane gets bigger:

o BP increases o MP increases o Density increases

Non-polar Water insoluble Soluble in non-polar solvents Less dense than water As alkene gets bigger:

o BP increases o MP increases o Density increases

More reactive than alkanes due to C=C bond

Types of Alkene ReactionsTypes of Alkene Reactions

• The double-bond in alkenes make them chemically reactive

• Four major types of alkene reactions– Addition– Elimination– Substitution– Rearrangement

C=C

Addition ReactionsAddition Reactions

• Two reactants add together to form a single new product that includes all original atoms.

• Double bond becomes a single bond.

A + B C

C C C CX--Y+

X Y

Addition ReactionsAddition Reactions

• Halogenation: Addition of a Halide (fluorine, chlorine, bromine, iodine).

CH2CH3CH2CH Br-Br

Br

+ CH2CH3CH2CH

Br1-butene 1,2-dibromobutane

CHCH3CH3CH

H

+

H2-butene butane

H2 CHCH3CH3CH

• Hydrogenation: Metal (Pt, Pd or Ni) catalyzed addition of hydrogen atoms to C=C bond.

Pt

Addition ReactionsAddition Reactions• Acid Rxn: (HCl, HBr, etc.)• The major product of this rxn is 2-bromopropane, due to:• Markovnikov’s rule: When H-X reacts with alkene, H goes to

C that already has the most H.

CHCH3CH2

Br+

HHBr

CHCH3CH2

H Br

CHCH3CH2

1-bromopropane

2-bromopropane

ClH

H

CH3

Br

CH3

HH

+

Addition ReactionsAddition Reactions• Hydration: Water may react with alkene in presence of acid

catalyst

CH2CH3CH + H-OHOH H

H2SO4 CH2CH3CH

• Rxn also follows Markovnikov’s rule• This method can be used for large scale production of

alcohols

Addition ReactionsAddition Reactions• Polymerization: Addition rxn where identical molecules join

together to form long chains.

• This rxn can produce very long, inert alkane chains, useful in manufacture of food storage containers.

(CH2=CH2)Heat, P

Catalyst(CH2=CH2) (CH2=CH2)

polyethylene

Addition ReactionsAddition Reactions• Copolymerization: Addition rxn with 2 different monomers.

Note: --CH=CH2 is known as a vinyl group

CHCH2 +catalyst

ClCCH2

Cl

ClCH2

Cl

CH CCH2

Cl

Cl

vinyl chloride vinylidine chloride Saran wrap

Elimination ReactionsElimination Reactions

• Single reactant becomes multiple products.• Single bond becomes a double bond.

A B + C

C C C C

X Y

+ X--Y

Substitution ReactionsSubstitution Reactions

• 2 reactants exchange parts, create 2 new products.

C + R--Y

X

C + R--X

Y

Rearrangement ReactionsRearrangement Reactions

• Single reactant reorganized into isomer.

C CH H

HCH3CH2

C CH

HCH3

CH3

Acid Catalyst (H+)

Geometry of AlkynesGeometry of Alkynes• Copolymerization: Addition rxn with 2 different monomers.

2p2s1s

Energy

2p2 sp1s

• Insoluble in water

• Less dense than water

• Low MP, BP


AromaticsAromatics• 6 C ring structures with alternating double bonds (benzene).• Everything that is not aromatic, is aliphatic (alkanes, alkenes,

alkynes).• Early problem chemists found was that benzene was not

reactive (recall that alkenes are reactive due to double bond).• Kekule proposed that double bonds alternated between 2

equivalent structures• electrons move around a conjugated pi bond system of rings• Stabilizes structure and makes it less reactive.

Naming Benzene DerivativesNaming Benzene Derivatives1. For single replacement (H some FG), cmpd named as a

benzene derivative.CH3

N+ O

-O

CH3 OH

CH3CH3OHO

propylbenzene nitrobenzene methylbenzene hydroxybenzene

isopropylbenzene carboxybenzene

Naming Benzene Derivatives (continued)Naming Benzene Derivatives (continued)

Some common names are IUPAC-accepted and used preferentially.


Naming Benzene DerivativesNaming Benzene Derivatives• Some benzene derivatives better known by common names

accepted by IUPAC.NH2

aniline commonaminobenzene IUPAC

• The benzene ring can also be an attached substituent group.

CH3 CH3

4-phenylheptane 1,1-diphenylcyclobutane

Naming Benzene DerivativesNaming Benzene Derivatives• With only 2 groups on benzene ring, can use o, m, p (ortho,

meta, para) nomenclature.

CH3

CH3

CH3

CH3

CH3

CH3

o- m- p-

p-methylanilinep-aminotoluene

m-dichlorobenzeneo-nitrotoluene

CH3 N+

O-

OCl

Cl

NH2

CH3

Naming Benzene DerivativesNaming Benzene Derivatives• When 2 or more groups are attached to benzene ring, positions can be

indicated by numbering ring C atoms to obtain lowest possible numbers.

Br

Br

Br Br

Cl Cl

Cl

CH3

OH

OOH

1,2,3,5-tetrabromobenzene 1,2,4-trichlorobenzene

3-hydroxy-4-methylbenzoic acid

This could be named as either a disubstituted:toluenephenolbenzoic acid

Naming AlkynesNaming Alkynes• Name of the compound ends in -yne.• The longest chain chosen for the root name must include both carbon atoms of

the triple bond.• The root chain must be numbered from the end nearest a triple bond carbon atom.

– If the triple bond is in the center of the chain, the nearest substituent rule is used to determine the end where numbering starts.

• The smaller of the two numbers designating the carbon atoms of the triple bond is used as the triple bond locator.

• If several multiple bonds are present, each must be assigned a locator number. Double bonds precede triple bonds in the IUPAC name, but the chain is numbered from the end nearest a multiple bond, regardless of its nature. The name will then have multiplier prefix (e.g., diyne, triyne, etc.)

• Because the triple bond is linear, it can only be accommodated in rings larger than ten carbons. In simple cycloalkynes the triple bond carbons are assigned ring locations #1 and #2. Which of the two is #1 may be determined by the nearest substituent rule.

• Substituent groups containing triple bonds are: HC≡C– Ethynyl group

33

CH3 CH3

Examples of AlkynesExamples of Alkynes

CH3

CH3

Br

CH3 CH3

CH3

CH3

BrCH

CH3

but-2-yne pent-2-yne hex-3-yne

2-bromohex-3-yne 5-bromohexa-1,3-diyne

Examples of AlkynesExamples of Alkynes

CH3

CH3

CH3

4-methyl-2-pentyne

1-cyclopentyl-1-butyneCH3

1,6-dichloro-2,2-dimethyl-3-hexyneCH3 CH3

ClCl

1-phenyl-1,4-pentadiyneCH

cis-1,2-diethynylcyclobutaneCHCH

CH3

CH3

2-hexyne

chem 1152: ch. 12 unsaturated hydrocarbons. types of hydrocarbons saturated: all c—c bonds are...

Documents

bond pi

multiple double bonds

slabaugh mr

pi bond bridge

number of double bonds

longest chain

attached groups

cc bonds