Alkenes, Alkynes and Dienes

Presented by:Janine V. SameloBSChem2

ALKENES Alkenes are a family of hydrocarbons

(compounds containing carbon and hydrogen only) containing a carbon-carbon double bond.

General formula: CnH2n

Example:

Functional group = carbon-carbon double bond

sp2 hybridization => flat, 120o bond angles

Shape => trigonal planar σ bond & π bond => H2C=CH2 C2H4 ethylene

ethene C2H4

propene C3H6

C CH

H H

H

ISOMERISM OF ALKENES

Structural isomerism All the alkenes with 4 or more carbon

atoms in them show structural isomerism. This means that there are two or more different structural formulae that you can draw for each molecular formula.

For example, with C4H8, it isn't too difficult to come up with these three structural isomers:

ISOMERISM OF ALKENES

Geometric (cis-trans) isomerism The carbon-carbon double bond doesn't

allow any rotation about it. That means that it is possible to have the CH3 groups on either end of the molecule locked either on one side of the molecule or opposite each other.

These are called cis-but-2-ene (where the groups are on the same side) or trans-but-2-ene (where they are on opposite sides).

Cis-but-2-ene is also known as (Z)-but-2-ene; trans-but-2-ene is also known as (E)-but-2-ene.

PHYSICAL PROPERTIES OF ALKENES

Boiling Points The boiling point of each alkene is very

similar to that of the alkane with the same number of carbon atoms. Ethene, propene and the various butenes are gases at room temperature. All the rest that you are likely to come across are liquids.

It has a boiling point which is a small number of degrees lower than the corresponding alkane. The only attractions involved are Van der Waals dispersion forces, and these depend on the shape of the molecule and the number of electrons it contains. Each alkene has 2 fewer electrons than the alkane with the same number of carbons.

PHYSICAL PROPERTIES OF ALKENES

Solubility Alkenes are virtually insoluble in water,

but dissolve in organic solvents. non-polar or weakly polar no hydrogen bonding relatively low mp/bp ~ (similar to

alkanes) water insoluble

Importance:

common group in biological molecules

starting material for synthesis of many plastics

NOMENCLATURE

1.Parent chain = longest continuous carbon chain that contains the C=C.

alkane => change –ane to –ene

• prefix a locant for the carbon-carbon double bond using the principle of lower number.

2.Alphabetize, name the substituents, etc.

3.If a geometric isomer, use E/Z (or cis/trans) to indicate which isomer it is.

SOURCES

Synthesis of AlkenesAlkyl halides are dehydrohalogenated with base to form alkenes.  Alcohols are dehydrated with heat and acid to form alkenes. The product with the most number of carbons attached to the carbon-carbon double bond is formed in the higher yield.

USES of ALKENES Polymers of Alkenes

Ethylene is polymerized to polyethylene, which is used for bags, films, and bottles.

Propylene is polymerized to polypropylene, which is used for plastics. 

Styrene is polymerized to polystyrene, which is used for plastics, plastic cups, and foam insulation. 

Methyl α-methacrylate is polymerized to polymethyl α-methacrylate, which is used for plexiglass and Lucite paints. 

Acrylonitrile is polymerized to polyacrylonitrile, which is used as Orlon or Acrylan fibers. 

Tetrafluoroethylene is polymerized to polytetrafluoroethylene, which is used as Teflon.

Vinyl chloride is polymerized to polyvinyl chloride, which is used in plastics, films, and plumbing.

Vinylidene chloride is polymerized to polyvinylidene chloride, which is used in Saran.

CHEMICAL REACTIVITY

addition reactions. Example

The rather exposed electrons in the pi bond are particularly open to attack by things which carry some degree of positive charge. These are called electrophiles

SYNTHESIS

3. dehalogenation of vicinal dihalides

| | | | — C — C — + Zn — C = C — + ZnX2

| | X X

example: CH3CH2CHCH2 + Zn CH3CH2CH=CH2 +

ZnBr2 Br Br Not generally useful as vicinal dihalides

are usually made from alkenes. May be used to “protect” a carbon-carbon double bond.

SYNTHESIS

1. dehydrohalogenation of alkyl halides | | | |— C — C — + KOH(alc.) — C = C — + KX + H2O | | H X

a) RX: 3o > 2o > 1o b) no rearragement c) may yield mixtures d) Saytzeff orientatione) element effectf) isotope effectg) rate = k [RX] [KOH]h) Mechanism = E2

SYNTHESIS

rate = k [RX] [KOH] => both RX & KOH in RDS

R-I > R-Br > R-Cl “element effect”

=> C—X broken in RDS

R-H > R-D “isotope effect”

=> C—H broken in RDS

Concerted reaction: both the C—X and C—H bonds are broken in the rate determining step.

SYNTHESIS

Mechanism = elimination, bimolecular E2

One step! “Concerted” reaction.

base:

C

W

C

H

C C + H:base + :WRDS

SYNTHESIS

CH3CHCH3 + KOH(alc) CH3CH=CH2

Br isopropyl bromide propylene CH3CH2CH2CH2-Br + KOH(alc) CH3CH2CH=CH2

n-butyl bromide 1-butene

CH3CH2CHCH3 + KOH(alc) CH3CH2CH=CH2

Br 1-butene 19% sec-butyl bromide +

CH3CH=CHCH3

2-butene 81%

DIENES

.

Hydrocarbon containing twocarbon-carbon double bonds

Alkadienes

Isolated dienes

1,4-pentadiene1,5-Cyclo-octadiene

Separated by oneor more sp3-C atom.Separated by oneor more sp3-C atom.Conjugated dienes:

1,3-butadiene

1,3-cyclo- hexadiene

Double bonds andsingle bonds alternatealong the chain.

Double bonds andsingle bonds alternatealong the chain.

Cumulated dienes

Allene

The C atom is commonfor two double bondsThe C atom is commonfor two double bonds

C C C C C

C C C C

H2C C CH2

Hydrocarbon containing twocarbon-carbon double bonds

NOMENCLATURE

.

cis,cis –2,4-hexadiene

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

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

cis,trans-

C CC C

CH3H3C

H

HH

HC C

C CCH3

H3C

HH

H

H

•Suffix: ne diene• Cis-trans isomers:

STRUCTURE OF DIENES

. 4 C atoms are sp2- hybridized.

C2-C3 σbond: sp2-sp2overlap C

H

H

C CC

H

HH

H

1,3-Butadiene:

πbond： 2p-2p overlap

C2-C3 partially overlap by 2p-2p orbital

4 C atoms are coplanar

CH

H

C CC

H

HH

H

4 πelectrons are delocalized over 4 C atomsC

Delocalization of πelectronslowers the energy.

CONFORMATION

.Two possible planar conformation of 1,3-butadiene:

s-Cis conformation s-Trans conformation

REACTIONS Conjugated dienes have enhanced stability as

compared to molecules without conjugated double bonds due to resonance. In general, this makes them slightly less reactive than other types of alkenes in general and dienes specifically. However, many reactions proceed through high-energy cation or radical intermediates; in these cases the resonance stabilization of the intermediate allyl species makes conjugated dienes more reactive than non-conjugated dienes or simple alkenes.

Hydrobromination: Example: Butadiene + HBr--> 3-bromobutene (Low

Temperature) + 1-bromobut-2-ene (High Temperature) + 1-bromobutene (Not Observed)

REACTIONS

Diels-Alder Reaction One of the most important of all diene

reactions is the Diels-Alder Reaction, in which a conjugated diene reacts with an dienophile to form a cyclohexene.

Requirements: The diene must be able to access the s-cis conformation for the reaction to take place.

Example:H2C CH C

O

H H2C CH C

O

OCH2CH3 H2C CH C N

HC C COOCH3

DINEOPHILES - - A Dienophile must contain a double or triple

bond. Typically, an electron withdrawing group is conjugated to the dienophile to make it electron-poor (nitriles, ketones, and esters are common electron withdrawing groups). Because the reaction is highly stereospecific, the configuration of the dieneophile will determine the relative stereochemistry of the cyclohexene product.

The Diels-Alder reaction occurs most effectively with an electron-poor dieneophile and an electron-rich diene ('normal demand'). 'Inverse demand' Diels-Alder reactions can also be carried out, in which the dienophile is electron-rich and the diene electron-poor.

A species which likes to attack Dienes

ALKYNES- PROPERTIES Alkynes contain carbon-carbon triple

bonds.  The carbon in an alkyne is sp, has a

bond angle of 180o, and a linear shape.  A carbon-carbon triple bond contains one sigma bond and two pi bonds. 

A terminal alkyne contains at least one hydrogen attached to the carbon-carbon triple bond.  An alkyne that is not terminal contains two alkyl groups attached to the carbon-carbon triple bond.

SYNTHESIS AND REACTION

Synthesis of AlkynesVicinal dihalides are dehydrohalogenated twice with base to form alkynes. Geminal dihalides are dehydrohalogenated twice with base to form alkynes.

Reactions of AlkynesHydrogen halide adds across a triple bond, via Markovnikov addition and with anti or syn addition, to form dihaloalkanes. Halogen adds across a triple bond to form a tetrahaloalkane.  Hydrogen adds across a triple bond to make an alkane.

PHYSICAL PROPERTIES Alkynes are compounds which have low

polarity, and have physical properties that are essentially the same as those of the alkanes and alkenes.

They are insoluble in water. They are quite soluble in the usual organic

solvents of low polarity (e.g. ligroin, ether, benzene, carbon tetrachloride, etc.).

They are less dense than water. Their boiling points show the usual increase

with increasing carbon number. They are very nearly the same as the

boiling points of alkanes or alkenes with the same carbon skeletons.

PREPARATION

Alkynes Preparation The carbon-carbon triple bond of the alkynes

is formed in the same way as a double bond of the alkenes, by the elimination of atoms or groups from two adjacent carbons.

  W X W XHC - CH ==> HC = CH ==>

HCCH X X Alkane Alkene

Alkyne

The groups that are eliminated and the reagents used are essentially the same as in the preparations of alkenes.

ALKYNE REDUCTION H3C C C CH3

xs H2

Pt

CH3CH2CH2CH3

H2/Pd/BaSO4

Quinoline(Lindlar's Catalyst)

H3CC

H

C

H

CH3

Na NH3 (liq)

H3CC

H

CH

CH3

H3C C C H

HXH3C C C H

H

+

X-

H3C C C H

H

X

Markovnikov addition

a vinyl halide

H3C C C H

H

X

HXH3C C C H

H

HX

+ X-

a heteroatom stabilized carbocation

H3C C C H

H

HX+

H3C C C H

H

X H

X

a geminal dihalide

HYDROGEN HALIDE ADDITION

HYDRATION H3C C C H

H2O/H2SO4

HgSO4H3C C

O

CH3

H3C C C H H+H3C C C H

H

+a vinyl cation

H2O

H+ H3C C C H

H

OHan enol

H3C C C H

H

OH

H3C C

O

CH3

enol ketone

Keto-enol tautomerization

HALOGENATION

Formation of Vicinal tetrahalides C C

an alkyne

X2 C C

X X

X X

a vicinal tetrahalide

carbontetrachloride

SUBSTITUTION R C CH

NaR C C Na

+

R C C Na+ + R'CH2 X R C C CH2R' + NaX

+ H2

TerminalAlkyne

Methylor

PrimaryAlkylHalide

Alkynyl Anion Synthesis of Alkynes via Bimolecular Nucleophilic Substitution

DIELS-ALDER REACTION

Six-membered Ring formation (4+2)

Diene + Dienophile

C

C

CC

C

C

CLEAVAGE REACTIONS hot KMnO4

R C C H R C

O

OH +

H C C Hozonolysis

2(reductive workup)

ozonolysis

(reductive workup)H C

O

OH

H C

O

OH

R C C H R C

O

OH +

H C C H

ozonolysis

2(oxidative workup)

ozonolysis (oxidative workup)

CO2

CO2

orhot KMnO4

or