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Alkanes can also form cyclic structures

CH2 CH2

CH2CH2

CH2 CH2

CH2

CH2 CH2

CH2

CH2CH2

CH2

CH2CH2

CH2

CH2

CH2

Cyclopropane Cyclobutane CyclopentaneCyclohexane

Can be conveniently represented using line segment formulae

General formula for cycloalkanes: CnH2n

C

CC

C

CC

H HH

H

HH

HHH

H

HH

NOT

cyclohexane benzene

Note:

CH3 CH3

CH3

Cycloalkane nomenclature can be extended to include substitution

Methylcyclohexane 1,3-Dimethylcyclohexane

Only one cycloalkane has a planar structure: cyclopropane

All others have non-planar structureH

CH

H

H

109.5o

Ideal tetrahedral angle is 109.5o

sp3 hybridised carbons with bond angles very different to

109.5o will be less stable (higher in energy)

Cyclopropane C

C C

60o

H

H

H

H

H

H

Bond angle approaching 60o

Cyclopropane is said to sufferfrom angle-strain

All C-H bonds in cyclopropane are eclipsed

Ball-and-stick model of cyclopropane

Cyclobutane also suffers from angle-stain, but to a lesser extent

HC

C

H H

H

H

H

HH

C

C

•Actual structure of cyclobutane is ‘butterfly-shaped’

•This avoids eclipsing of hydrogens on neighbouring carbons

Cyclopentane has almost zero angle-strain

To relieve torsional strain due to eclipsed C-H bonds,cyclopentane relaxes into a non-planar structure

HH H

H

HH

H

H

H HOne CH2 group out of the plane of the ring

Cyclohexane

A planar structure would have internal bond angles of 120o and eclipsed C-H bonds

120o

Actual structure relaxes into a chair conformationThis reduces the bond angle to 109o

~109o

Geometry about each Carbon very close to tetrahedral ideal•Angle strain ~ zero

H

HH

H

H

H

HH

H

H

H

H

All C-H bonds staggered, i.e. torsional strain ~ zero

Newman projection along any C-C bond

H

H

H

H

H

HH

H

H

H

HH

H

H

H

H

The chair conformation contains two different hydrogen environments

H

HH

H

H

H

HH

H

H

H

H

6 Axial Hydrogens 6 Equatorial Hydrogens

The chair conformation of cyclohexane, shown in ball-and-stick and space-filling

models.

At temperatures below 230 K (-43C):•can observe that two different types of hydrogen environment are present on cyclohexane

Above this temperature, observe only one hydrogen environment

Reason: cyclohexane molecules are not static above 230 Ki.e. exist in different conformations

Undergo ring inversion

Boat conformationExists in trace quantities

H

HH

H

H

H

HH

H

H

H

H H

HH

H

H

H

H

H

H

H

H

HH

H

H

HH

H

H

H

H

H

HH

Note: hydrogens axial in one chair conformation equatorial in the other

Ball-and-stick model of boat cyclohexane

Energy barrier to ring inversion = 43 kJ mol-1

Too low to prevent ring inversion at 25C

Rate of ring inversion at 25C = 2 x 105 s-1

Hence, observe only one time averaged hydrogen environment at 25C

Both chair conformations equal in energy

Hence, samples of cyclohexane at 25C consist of 50:50 mixtures of the two chair conformations[plus trace quantity of the boat conformation]

What if one of the cyclohexane hydrogens were replaced by a methyl group?

= H

HH

H

H

H

H

H

H

H

H

H

Cyclohexane

Methylcyclohexane

CH3

The two chair conformations are no longer equivalent

One has the methyl group in an axial position; one in an equatorial position

H

HH

H

H

H

CH3

H

H

H

H

H

H

HH

H

H

H

HH

H

H

H

CH3

These interconvert by ring inversion (exist in equilibrium)

[Inversion proceeds through boat conformations which exist in trace amounts]

Can simplify diagram by omitting the C-H bonds

CH3

CH3

Methyl axial Methyl equatorial

In methylcyclohexane, the axial and equatorial conformations are not equivalent

[Differ in energy] CH3

Methyl axial

View along this bond

Newman projection would look like:

Compare:

CH3

H H

CH3

H

HGauche conformation of butane3.8 kJ mol-1 steric strain energy

H

H CH2

CH3

CH2H

CH2

CH2

Gauche interaction3.8 kJ mol-1 steric strain energy

The steric interactions between the axial methyl and the 1,3-diaxial hydrogens are evident in the ball and

stick structure.

Two gauche interactions exist in axial methylcyclohexane

CH3H

HH

H

H

Contribute 2 x 3.8 kJ mol-1

= 7.6 kJ mol-1 steric strain energy

CH3

CH3

7.6 kJ mol-1

Strain energyNo strain energy

Difference in strain energy the only difference between the conformations

Hence, G for ring inversion of methylcyclohexane = -7.6 kJ mol-1

Relationship between free energy (G) and equilibrium

constant (Keq) at equilibrium

G = - RTlnKeq

Hence, at 25C (298K) [and with R = 8.314 J mol-1 K-1]

Keg = exp8.314 J mol-1 K-1 x 298 KRT

- G=

7600 J mol-1exp

= exp [ 3.07] = 21.5

Hence, the equilibrium constant (Keq) for

CH3

CH3

At 25C = 21.5

i.e. Keg = 21.5 =[methyl axial]

[methyl equatorial]

4% axial

96% equatorial

Sources of alkanes

•Lower Mol. Mt. (~ < 5 Carbons): natural gas

•Larger Mol. Wt.: petroluem of crude oil

Crude oil: complex mixture of hydrocarbons

Separated into fractions based on boiling point ranges

Boiling point related to molecular weight, i.e to number of carbons

•< 5 Carbons: gases at room temperature

•5 Carbons < ~18 Carbons: liquids at room temperature

•> 18 Carbons: solids at room temperature

•Increasing molecular size results in increasing tendency to form condensed phases

•Associated with weak intermolecular interactions between alkane molecules

•London dispersion forces: weak electrostatic attractions between induced dipoles, i.e. are…

•Van der Waals’ forces between electrons of one molecule and nuclei of another

•Extent of attraction increases with increasing molecular size

•Weak interactions compared to hydrogen bonding or ionic bonding

Solubility of alkanes

•‘Like dissolves like’: alkanes soluble in other alkanes, e.g petroleum

•[Soluble: single liquid phase results upon mixing]

•Alkanes insoluble in water, i.e are hydrophobic

•Mixtures with water separate into two liquid phases: aqueous and hydrocarbon

Reactions of alkanes

•Relatively inert; contain only stable C-C and C-H bonds

•Some important reactions:

1. Combustion, e.g.

2 C4H10 + 13 O2 → 8 CO2 + 10 H2O

H = - 2877 kJ mol-1 i.e. exothermic

2. Steam reforming

CH4 + H2O → 3H2 + CO

↓N2

NH3

↓

CO2+ → Urea

3. Reaction with halogens

CH4 + Cl2

CH3Cl + HClChloromethane(Methyl chloride)

Heat or light

CH2Cl2Dichloromethane

With excess Cl2

Cl2

CHCl3Chloroform

(Trichloromethane)

Cl2CCl4Tetrachloromethane

(Carbon tetrachloride)

4. Catalytic cracking

•Fragmentation of alkanes into smaller molecules, e.g:

CH3 CH2 CH2 CH2 CH2 CH3

~ 500oC

Catalystsurface

CH2 CH2

+ CH3 CH CH2

+ others + H2

•The products of these reactions are a new type of hydrocarbon

•They are said to be ‘unsaturated’ compared to alkanes

•i.e., have fewer Hydrogens per Carbon than alkanes, which are said to be ‘saturated’

Unsaturated hydrocarbons contain Carbon-Carbon multiple bonds

Classes of unsaturated hydrocarbons are defined by the types of Carbon-Carbon multiple bonds they contain

Alkenes: contain Carbon-Carbon double bonds

Alkynes: contain Carbon-Carbon triple bonds

C C Carbon-Carbon double bond

C C Carbon-Carbon triple bond

Carbon valency of four maintained in alkenes and alkynes

Alkenes Older name: Olefins

Characterised by presence of Carbon-Carbon double bonds

Generalstructuralformula

C CR

R

R

R

Where ‘R’ = Hydrogen

or alkyl group

Two Carbons and all four ‘R’ groups are lying on the same plane

Bond angles about each Carbon ~ 120o

C CR

R

R

R

C CR

R

R

R

120o

120o

sp3 hybridisation cannot provide the geometry found at Carbon in Carbon-Carbon double bonds

1s2s 2p 2p 2p

Alternative hybridisation required

Hybridisation

1ssp2 sp2 sp2

pz

2e-

1e- 1e- 1e-

1e-

Three sp2 hybridised orbitals can be arrayed to give trigonal geometry

120o 120o

120o

The remaining 2pz orbital is orthogonal to the three sp2 orbitals

2pz orbital

View along z axis

2pz orbital

View along xy plane

bond formation results from overlap of two sp2 hybridised orbitals

[A -antibonding orbital is also formed, but this is not occupied by electrons]

Overlap of the pz orbitals results in formation of a bond

[A -antibonding orbital is also formed, but this is not occupied by electrons]

orbital: has a nodal plane on which lies on the bond axis

electron density lies above and below the plane containing the two Carbons and four ‘R’ groups

C CRR

R R CR RView along the Carbon-Carbon bond

Note: constitutes one molecular orbital

i.e. constitutes one bond when occupied

C CCarbon-Carbon double bond:

•One bond; One bond

•Both occupied by two electrons

The bonding in ethene consists of one sp2-sp2 carbon-carbon σ bond, four sp2-s carbon-hydrogen

σ bonds, and one p-p π bond.

Rotation of one sp2 carbon 90° with respect to another orients the p orbitals perpendicular to

one another so that no overlap (and therefore no pi bond) is possible.

•Rotation about a Carbon-Carbon double bond requires

opening up of the bond

•Requires large input of energy (~ 268 kJ mol-1)

•Hence, rotation about C=C bonds does not occur at room temperature•Consequently, a new form of isomerism becomes possible for alkenes

•Consider an alkene with one Hydrogen and one alkyl group ‘R’ bonded to each Carbon

•Two structures are possible

C CR

H

R

H

or C CH

R

R

H

•This form of isomerism is known as Cis-Trans isomerism•[older term: geometrical isomerism]

•The cis isomer is that with like groups on the same side of the

C=C•The trans isomer is that with like groups on opposite sides

of the C=C

C CR

H

R

HC C

H

R

R

H

Cis isomer Trans isomer

First two members of the alkene series:

C CH

H

H

H

Ethene(Ethylene)

C CCH3

H

H

HPropene

(Propylene)

Note:C C

CH3

H

H

HC C

H

CH3

H

H= = CH3 CH CH2

Nomenclature:

•Prefix indicates number of carbons•(‘eth…’ = 2C; ‘prop…’ = 3C; etc.)

•Suffix ‘…ene’ indicates presence of C=C

ButeneC C C C1 2 3 4 Could have C=C between C1 and C2

or between C2 and C3

1-Butene

CH2 CH CH2 CH3

1 2 3 4

CH3 CH CH CH3

1 2 3 4

2-Butene

Note:

CH2 CH CH2 CH3 = CH3 CH2 CH CH2 = 1-Butene

3. Number indicates starting point of the C=C, i.e. number

through the C=C

1. 1-Butene and 2-butene are structural isomers

2.

4. Cis-Trans isomerism is possible for 2-butene

•There are two isomeric 2-butenes

C C

CH3

H

H

CH3

C C

CH3

H

CH3

H

Trans-2-butene

b.p. 3.7oC

m.p. -139oC

Cis-2-butene

b.p. 0.3oC

m.p. -106oC

Some other alkenes

CH2 C CH2 CH3

CH3

1 2 3 4

2-Methyl-1-butene

CH3 CH CH CH

CH3

CH3

1 2 3 4 5

4-Methyl-2-pentene

CH2 CH CH CH

1 2 3 4 5CH3

1,3-Pentadiene

C

C

H

H CH2CH2CH3

CH2CH3

123

45 6 7

Cis-3-heptene

Trans-2-decene

Can have cycloalkenes

CyclopenteneCyclohexene

3

CH3

1

24

53-Methylcyclopentene

1

23

4

56

1,4-Cyclohexadiene

Note:

=C

CC

C

CC H

HH

H

H H

H H

Lycopene molecular structure

Reactivity of alkenes

•More reactive than alkanes•Due to the presence of C=C

Energy-level diagram for ethene CH2=CH2

CC

*CC

CC

*CC

CH x 4

*CH x 4

Energy

C C

H

H

H

H

electrons in alkenes are available to become involved in bond formation processes

Essential processes in the synthesis of new molecules:

formation of new covalent bonds

Covalent bonds: pairs of electrons shared between nuclei (atoms)

In the synthesis of organic molecules, a major strategy for forming new covalent bonds is:

donation of an electron pair by one molecular species…

…to form a covalent bond with another, electron deficient molecular species

Electron pair donating species are known as nucleophiles

Electron pair accepting species are known as electrophiles

Reaction of a nucleophile with an electrophile results in the formation of a new covalent bond

Alkene hydrogenation

•Addition of hydrogen (H2) across a C=C

General reaction

C CR

R

R

R

H2

Catalyst

C CRR

R

R

H H

•Alkene bond is lost, and two new C-H bonds formed•Alkene converted to alkane•No reaction in absence of catalyst•Typical catalysts: Palladium (Pd), Platinum (Pt), Nickel (Ni), Rhodium (Rh) or other metals•Catalysts usually supported on materials such as charcoal•E.g. Pd/C “Palladium on Carbon”

Examples

H2 (g)

CH2 CH CH2 CH2 CH2 CH3

Pt/C

CH3 CH2 CH2 CH2 CH2 CH3

1-Hexene Hexane

2 H2 (g)

CH2 CH CH CH CH2 CH3

Pt/C

CH3 CH2 CH2 CH2 CH2 CH3

1,3-HexadieneHexane

CH2 C

CH3

CH2 CH3

H2 (g)

Pt/C

CH3 C

CH3

CH2 CH3

H2-Methyl-1-butene2-Methylbutane

•Reaction occurs at the catalyst surface

•H2 molecules adsorbed onto catalyst surface

•Both Hydrogens added to same face of C=C

CH3

CH3

H2 (g)

Pt/C

CH3

CH3

H

H1,2-Dimethylcyclohexene

Cis-1,2-dimethylcyclohexane

•Both Hydrogens added to the same face of the cyclohexene

C=C•[Cis/Trans naming system can be extended to cyclic systems]

Addition of HX to alkenes

General reaction

C CR

R

R

R

C CRR

R

R

H XHXX = Cl, Br, I

•C=C bond lost; new C-H and C-X bonds formed

e.g:HCl

CH2 CH CH3 CH3 CH

Cl

CH3

Propene2-Chloropropane

(only product)

H2C CH2 CH3

Cl

1-Chloropropane(not formed)

•To explain this, need to consider the reaction mechanism

Reaction mechanism:

•detailed sequence of bond breaking and bond formation in

going from reactants to products•Addition of HX to alkenes: reaction involves two steps

1st Step: Addition of proton (H+)

2nd Step: Addition of halide (X-)1st Step

C C

H

C CH

•Alkene electrons

attack proton

•New C-H bond results

•Remaining Carbon short 1 electron•Carbon positively charged

•Addition of H+ to the alkene bond forms a new C-H

bond and a carbocation intermediate

•[or carbonium ion]

2nd Step

Halide ion attacks electron deficient carbon

C CH

X

C C

HX

New C-X bond results

•Reaction involves two steps with an intermediate species

•Each step proceeds through a transition state

Reaction Coordinate

Energy

ReactantsProducts

TransitionState

1Transition

State2

Intermediate

Reaction of HCl with CH3-CH=CH2

1st Step: addition of H+ to form a carbocation intermediate

Two possible modes of addition

CH CH2CH3

HCH3 CH CH3

or

CH CH2CH3

H

CH3 CH2 CH2

I.e. two possible carbocation intermediates

Classification of carbocations

R C H

HPrimary (1o)Carbocation

R C H

R

Secondary (2o)Carbocation

R C R

RTertiary (3o)Carbocation

CH3 CH CH3

2o Carbocation

CH3 CH2 CH2

1o Carbocation

The relative order of stability for carbocations is:

Most stable 3o > 2o > 1o Least stable

•This is because carbocations can draw electron density along bonds; known as an inductive effect

•This effect is significant for alkyl substituents, but weak for Hydrogens

R C H

H

R C H

R

R C R

R

> > > <> >

Most stabilisedLeast stabilised

Addition of HCl to CH3-CH=CH2 proceeds so as to give the

more stable of the two possible carbocation intermediates, i.e:

CH CH2CH3

HCH3 CH CH3

CH3 CH2 CH2

Not formed

Addition of chloride then gives 2-chloropropane exclusively

Cl-

CH3 CH CH3

Cl

Additions of HX to alkenes which follow this pattern are said to obey Markovnikov’s rule

“Reaction proceeds via the more stable possible carbocation intermediate”

Reaction energy diagram for formation of the isopropyl and propyl cations from propene

Other examples

CH3 C

CH3

CH2

HBrCH3 C

CH3

CH3

Br2-Methylpropene2-Bromo-2-methyl-

propane

not CH3 C

CH3

CH2

H

Br

1-Bromo-2-methyl-propane

CH3 HCl CH3

Cl

1-Methylcyclohexene 1-Chloro-1-methyl-cyclohexane

notCH3

ClH

H

1-Chloro-2-methyl-cyclohexane

CH CHCH3 CH3

HCl

CH2 CHCH3 CH3

Cl

2-Butene 2-Chlorobutane

CH CH2CH3 CH3

Cl

Same structure

(Symmetrical alkene)

Addition of water to alkenes

•Follows same pattern as addition of HX•Acid catalysis required

CH CH2CH3 + H2OH catalyst

CH3 CH CH3

OH

Propene 2-Hydroxypropane(2-Propanol)

Mechanism:

CH CH2CH3

HCH3 CH CH3

1. Protonation of C=C so as to give the more stable carbocation intermediate

2. Attack on the carbocation by water acting as a nucleophile

CH3 CH CH3

OH

H

CH3 CH CH3

OH H

3. Loss of proton to give the product and regenerate the catalyst

CH3 CH CH3

OH H

CH3 CH CH3

OH + H

•Acid catalysed addition of water often difficult to control•A Mercury (II) mediated version often used - oxymercuration

CH3

i) (CH3CO2)2Hg, H2O

ii) NaBH4(Sodium borohydride)

CH3

OH

•Gives exclusively Markovnikov addition

Hydroboration

1-Methylcyclopentene1-Hydroxy-1-methyl-

cyclopentane

CH3

i) "BH3" (Borane)

ii) H2O2, NaOH

CH3

OHH

H1-Methylcyclopentene

1-Hydroxy-2-methyl-cyclopentane

•Gives exclusively anti-Markovnikov addition

Mechanisms of these reactions beyond the scope of this module

Alkene hydroxylation

C CR

R

R

R

KMnO4

or OsO4

C C RR

R

R

HO OH

•Alkene bond lost; two new C-OH bonds formed

Alkene epoxidation

C CR

R

R

R

RCO3H

C CR

R

R

R

O

(Peroxy acids)Epoxides

•Alkene bond lost; two new C-O bonds are formed to the same Oxygen

Examples

CH CH2CH3

OsO4

CH CH2CH3

OH OH

Propene

CH CH2CH3

CH3CO3H (Peroxyacetic acid)

O

Propane-1,2-diol

1,2-Epoxypropane

Cyclopentene

Cis-1,2-cyclopentanediol

OsO4H

H

H

H

OH

OH

CH3CO3HH

H

O

1,2-Epoxycyclopentane

Ozonolysis of alkenes

•Ozone (O3): strong oxidising agent

•Adds to C=C with loss of both the and bonds•Products formed are known as ozonides

C CR

R

R

R

O3

OC

OO

CR

R

R

R

Ozonide•Ozonides usually not isolated, but further reacted with reducing agents

OC

OO

CR

R

R

R

ZnC C

R

R

R

ROO +

•Formation of two molecules each containing C=O (Carbonyl) groups

Overall process:

C CR

R

R

R

i) O3

ii) Zn

C CR

R

R

ROO +

Examples

CH CH2CH2CH3

i) O3

ii) Zn

CH OCH2CH3 O CH2+

1-Butene Aldehydes

C C

CH3

CH3

CH3

CH3

i) O3

ii) Zn

C

CH3

CH3

O2

2,3-Dimethyl-2-butene Ketone