Structure of Alkenes• Alkenes (and alkynes) are unsaturated hydrocarbons• Alkenes have one or more double bonds• The two bonds in a double bond are different:

- one bond is a sigma () bond; these are cylindrical in shape and are very strong

- the other is a pi (π) bond; these involve sideways overlap of p-orbitals and are weaker than bonds

• Alkenes are flat and have a trigonal planar shape around each of the two C’s in a double bond

Structure of Alkynes• Alkynes have one or more triple bonds• A triple bond consists of one bond and two π bonds

- the two π bonds are orthogonal (perpendicular)• Alkynes are linear around each of the two C’s in the

triple bond• Because alkenes and alkynes have π bonds, which are

much weaker than bonds, they are far more chemically reactive than alkanes

Naming Alkenes and Alkynes• Parent name ends in -ene or -yne• Find longest chain containing double or triple bond• Number C’s starting at end nearest multiple bond• Locate and number substituents and give full name

- use a number to indicate position of multiple bond

- cycloalkenes have cyclo- before the parent name; numbering begins at double bond, giving substituents lowest possible numbers

- use a prefix (di-, tri-) to indicate multiple double bonds in a compound

Cis-Trans Isomers of Alkenes• The π bond gives an alkene a rigid structure• Free rotation around the C-C bond is not possible

because the π bond would have to break and re-form• So, groups attached to the double bond are fixed on

one side or the other• If each C in the double bond has two different groups

attached, then cis-trans isomers are possible:- Cis = 2 groups attached to the same side of the double bond- Trans = 2 groups attached to opposite sides of the double bond

Addition Reactions of Alkenes and Alkynes• Addition (combination) reactions have the form

A + B AB• For alkenes the general reaction has the form

R2C=CR2 + A-B R2AC-CBR2

(where R = any alkyl group or H)• Addition reactions are the most common types of

reactions for alkenes and alkynes• The π bonds are easily broken, and that pair of

electrons can form a new bond• The reactions are favorable because the products (all

bonds) are more stable than the reactants

Hydrogenation of Alkenes and Alkynes• H2 can be added to alkenes or alkynes to form alkanes• Usually a metal catalyst (Pt, Pd or Ni) is used to speed up the

reaction (the reaction generally doesn’t work without a catalyst)• Because these reactions take place on a surface, hydrogenation

of substituted cycloalkenes produces cis products.

H2C CH2 + H2

Examples:

HC CH + 2H2

CH3

CH3

+ H2

H

H

H

H

H

H

H

H

H

H

H

H

CH3

CH3H

H

Hydrohalogenation of Alkenes• Hydrogen halides (HCl, HBr or HI) can add to alkenes to form

haloalkanes• When a hydrogen halide adds to a substituted alkene, the halide

goes to the more substituted C (Markovnikov’s rule)Examples:

C C

H

H H

H

+ HBr C C

H

H

H

Br

H

H

C C

H

H H

CH3

+ HCl C C

H

H

H

Cl

CH3

H

CH3

H

I

CH3

H

H

+ HI

Mechanism of hydrohalogenation

• Hydrohalogenation takes place in two steps• In the first step, H+ is transferred from HBr to the alkene to

form a carbocation and bromide ion• Second, Br- reacts with the carbocation to form a bromoalkane

Example:

C C

H

H H

CH3

+ C C

H

H

H

CH3

H

+ Br

C C

H

H

H

CH3

H

+ Br C C

H

H

H

Br

CH3

H

H Br

Addition of Water to Alkenes• In the presence of a strong acid catalyst (HCl, H2SO4 etc.)

alkenes react with H2O to form alcohols• Recall that acids form H3O+ in water; it is the H3O+ that reacts

with the alkene• Hydration reactions follow Markovnikov’s rule

Examples:

C C

H

H H

H

AcidCat.

C C

H

H

H

OH

H

H+ H2O

C C

H

H CH3

H

AcidCat.

C C

H

H

H

OH

H

CH3+ H2O

CH3

H

AcidCat.

+ H2O

CH3

OH

H

H

Mechanism of Acid-Catalyzed Alkene Hydration• First, the alkene reacts with H3O+ to form a carbocation• Next an H2O quickly reacts with the carbocation to form a protonated alcohol• In the last step the proton is removed by an H2O to form an alcohol

C C

H

H CH3

H

+ OH

H

H

C C

H

H

H

CH3

H

C C

H

H

H

CH3

H

+ OH H

C C

H

H

H

O

H

CH3

H

H

C C

H

H

H

O

H

CH3

H

H

+ OH H

C C

H

H

H

O

H

CH3

H

+ OH

H

H

+O

H H

Halogenation of Alkenes and Alkynes• Halogens (Cl2 or Br2) can add to alkenes or alkynes to form

haloalkanes• Alkenes form dihaloalkanes; alkynes form tetrahaloalkanes• Reaction with cycloalkenes produces a trans product

Examples:

C C

H

H H

H

C C

H

Br

H

H

Br

H+ Br2

+ Br2

Br

Br

C C CH3H + 2Br2 C C

Br

Br

H

Br

Br

CH3

Mechanism of Bromonation of Ethene• First, a Br+ is transferred from Br2 to the alkene to form a

bromonium ion and a bromide ion• Next, the bromide ion reacts with the bromonium ion to form

the product

C C

H

H H

H

+ Br Br C C

BrH

H

H

H

C C

BrH

H

H

H

+ Br

+ Br C C

H

Br

H

H

Br

H

Polymers

• A polymer is a long chain of repeating subunits called monomers- examples of natural polymers: DNA, protein, starch- example of synthetic polymers: polyethylene

• Many synthetic polymers are made from alkenes, although other functional groups are also used

• The monomers are added to the chain through a series of addition reactions

• Polymerization reactions usually require high temperature and pressure and are often radical reactions carried out with a catalyst

Conjugated Alkenes and Aromatic Compounds

• Recall that a double bond consists of one bond and one bond; a bond is formed by sideways overlap of two p orbitals (one electron comes from each orbital)

• A conjugated alkene has alternating double and single bonds• The p orbitals overlap in a conjugated system (the electrons

are “delocalized” throughout the system), making conjugated alkenes more stable than non-conjugated alkenes

• An aromatic hydrocarbon consists of alternating double and single bonds in a flat ring system

• Benzene (C6H6) is the most common aromatic hydrocarbon• In benzene all the double bonds are conjugated, and so the

electrons can circulate around the ring, making benzene more stable than 1,3,5-hexatriene (the p orbitals on the end of a chain can not overlap)

1,3,5-hexatriene

Resonance Structures• There are two ways to write the structure of benzene• These are called “resonance structures”• However, neither of these represents the true structure of

benzene since benzene has only one structure, with all C-C bonds being equivalent

• The true structure is a hybrid of the the two resonance structures; this can be represented by drawing the bonds as a circle

• We use the individual resonance structures when we write reaction mechanisms involving benzene to show more clearly the bond formation and bond breaking in the reaction

Naming Monosubstituted Benzene Compounds• Benzene compounds with a single substituent are named by

writing the substituent name followed by benzene• Many of these compounds also have common names that are

accepted by IUPAC (you should know those listed here)

CH3 OH NH2 OCH3

CO H

CO OH

Toluene(methylbenzene)

Phenol(hydroxybenzene)

Analine(aminobenzene)

Anisole(methoxybenzene)

Benzaldehyde(benzenecarbaldehyde

Benzoic Acid(benzenecarboxylic acid)

Styrene(phenylethene)

Naming Multisubstituted Benzene Compounds• When there are 2 or more substituents, they are numbered to

give the lowest numbers (alphabetical if same both ways)• Disubsituted benzenes are also named by the common prefixes

ortho, meta and para

Br

Br

CH3

Cl

OH

F

NH2

Br

meta-dibromobenzene(1,3-dibromobenzene)

ortho-chlorotoluene(1-chloro-2-methylbenzene)

para-ethylphenol(1-hydroxy-4-ethylbenzene)

4-bromo-2-fluoroanaline(1-amino-4-bromo-2-fluorobenzene)

Examples:

Physical Properties of Aromatic Compounds

• Because aromatic compounds (like benzene) are flat, they stack well, and so have higher melting and boiling points than corresponding alkanes and alkenes (similar to cycloalkanes)

• Substituted aromatic compounds can have higher or lower melting and boiling points than benzene- para-xylene has a higher m.p. than benzene- ortho and meta-xylene have lower m.p.’s than benzene

• Aromatic compounds are more dense than other hydrocarbons, but less dense than water (halogenated aromatics can be more dense than water, as can haloalkanes)

• Aromatic compounds are insoluble in water, and are commonly used as solvents for organic reactions

• Aromatic compounds are also flammable, and many are carcinogenic

Chemical Reactivity of Aromatic Compounds• Aromatic compounds do not undergo addition reactions because

they would lose their special stability (aromaticity)

• Instead, they undergo substitution reactions, which allow them to retain their aromaticity

• We will study three types of substitution reactions of benzene: halogenation, nitration and sulfonation

+ Br 2

Br

Br

Aromatic Loses aromaticity

+ Br 2

BrFeBr3

Aromatic Retains Aromaticity

+ HBr

Addition:

Substitution:

Halogenation of Benzene and Toluene• Br2 or Cl2 can react with benzene, using a catalyst, to form

bromobenzene or chlorobenzene• Only the monohalogenation product is produced• When Br2 or Cl2 reacts with toluene, a mixture of isomers

is produced- Ortho and para isomers are the major products, and meta isomer is the minor product

+ Cl2

ClFeCl3

+ HCl

CH3

+ Br2

FeBr3

CH3

Br

CH3

Br

CH3

Br

+ +

(Minor)

Examples:

+ HBr

Mechanism of Bromonation of Benzene• First, a Br+ is transferred from Br2 to benzene, forming a

carbocation and a chloride ion• Next, the chloride ion removes an H+ from the carbocation

to form chlorobenzene and HBr

+ Br BrFeBr3

+ Br

+ Br

HBr

HBr

+ H Br

Br

Nitration and Sulfonation of Benzene• Nitric acid can react with benzene, using sulfuric acid as a

catalyst, to form nitrobenzene plus water• First H2SO4 donates a proton to HNO3, which then

decomposes to form H2O and NO2+ (the reactive species)

• Sulfur trioxide plus sulfuric acid (fuming sulfuric acid) can react with benzene to produce benzenesulfonic acid

• First H2SO4 donates a proton to SO3 to produce HSO3+ (the

reactive species)

+ HNO3

NO2H2SO4

+ H2O

+ SO3

SO3HH2SO4