Alkanes, Alkenes and Halogenoalkanes

The only reactions of importance at normal temperatures are: Reactions of Alkanes Alkanes are relatively unreactive. The only reactions of importance at normal temperatures are: with oxygen (combustion) the reaction is highly exothermic and so alkanes are widely used as fuels. with the halogens chlorine and bromine

The Alkanes and halogens with bromine (Br2) with chlorine (Cl2) reaction is relatively slow reaction is faster reaction still requires light occurs explosively reaction requires sunlight Both produce hydrogen halides The reaction mechanism involves a FREE RADICAL CHAIN REACTION. FREE RADICAL = atom or molecule with an unpaired electron

H3C -H The Alkanes and halogens Reaction mechanism – alkane + bromine. R – H will represent the alkane R is an alkyl group e.g. in methane R will be methyl – CH3 H3C -H

Free radical chain reaction - Revision steps Initiation Propagation x 2 Termination

Br2  Br + Br Free radical chain reaction Initiation The first stage in the reaction. Light (hn) supplies the energy to break the Br – Br bond RESULT Two bromine atoms each with an unpaired electron. Br2  Br + Br Homolytic fission

R-H + Br  R + HBr Free radical chain reaction Another FREE RADICAL Propagation 1 The second stage in the reaction. Br are very reactive. There is a strong tendency for unpaired electrons to pair up. RESULT Bromine atom removes H atom from an alkane molecule . R-H + Br  R + HBr Another FREE RADICAL

R + Br-Br  R-Br + Br Free radical chain reaction Propagation 2 R reacts with a bromine molecule. R + Br-Br  R-Br + Br Br can then attack another alkane molecule This propagates the chain reaction. The two propagation steps continue in turn until one reactant is used up.

The chain is ended when any two free radicals meet up and combine. Free radical chain reaction Termination The chain is ended when any two free radicals meet up and combine. R + R  R-R Br + Br  Br-Br R + Br  R-Br

Free radical chain reaction Initiation Propagation Termination Light is needed to start this reaction but once it has started it continues in the dark. PRODUCTS Bromoalkane Hydrogen bromide

Free radical chain reaction Initiation Propagation Termination It is possible for multiple substitutions to take place. To prevent this (ensure monosubstitution) a large excess of alkane is used. Boiling point of the bromoalkane is higher than that of the alkane this means the excess alkane can easily be removed by distillation. The reaction is not specific - any hydrogen atom in any alkane can be substituted. The more complex the alkane the more complex the mixture of products.

Saturated hydrocarbon (Single bonds) Free radical chain reaction Initiation Propagation Termination Useful memory aid – the ‘S’ s Saturated hydrocarbon (Single bonds) Slow, Substitution reaction, takes place in Sunlight

Mostly ADDITION REACTIONS across double bond  saturated products Reactions of alkenes Mostly ADDITION REACTIONS across double bond  saturated products Hydrogen (H2) Hydrogen halides (e.g. HBr) Halogens (e.g. Br2)

Reactions of alkenes ADDITION of Hydrogen (H2) slightly exothermic so slow at room temperature  unobservable Carried out at high temperature with a catalyst Catalysts = nickel, platinum, palladium Heterogeneous catalysts – adsorb Hydrogen onto the surface

Cl2 reacts more vigorously and I2 reacts less vigorously. Reactions of alkenes ADDITION of Halogens (e.g. Cl2, Br2 ) Br2 Rapid decolourisation even in the absence of light Used to identify the presence of C = C Cl2 and I2 Also add across C = C. Which of these additions is more vigorous than that of Br2? Cl2 reacts more vigorously and I2 reacts less vigorously.

H Br2 C Br C H Reactions of alkenes ADDITION of Halogens (e.g. Cl2, Br2 ) Only one product C H Br2 C H Br

H H C Br C H H Reactions of alkenes δ+ δ- ADDITION of Br2 - likely mechanism – step 1 Bromine attacks electron rich C=C bond C=C bond electrons repel the Br2 electrons – temporary polarisation H H C δ+ δ- Br C electrophilic H H

electron deficient Br atom is attacked by the pair of π bond electrons Reactions of alkenes ADDITION of Br2 - likely mechanism Br-Br bond breaks heterolytically – both bond electrons go with the leaving Br- H H C δ+ δ- Br Br C electron deficient Br atom is attacked by the pair of π bond electrons H H

+ H Br C Br Reactions of alkenes ADDITION of Br2 - likely mechanism Its formation is the rate determining step the BROMONIUM ION Stabilised by delocalisation Evidence suggests the intermediate is a positive cyclic ion

+ Br H H C Br C Br Reactions of alkenes 1,2 - dibromoethane ADDITION of Br2 - likely mechanism – step 2 Rapid nucleophilic attack by the bromide ion on one of the carbon atoms of the bromonium ion Br H C C H Br + Br 1,2 - dibromoethane

+ Reactions of alkenes H Br H C C H Br H Br C C Cl ADDITION of Br2 in the presence of sodium chloride some 1- bromo - 2 - chloroethane is also produced H Br C H C Br2 Br2 with Cl- present H Br C H Br C Cl +

Addition of Cl2 to C = C occurs in a similar way. Reactions of alkenes ADDITION of Br2 in the presence of sodium chloride some 1- bromo - 2 - chloroethane is also produced C H Br + Cl Looks as though Cl- can compete with Br- as a nucleophile and intercepts the cyclic ionic intermediate. Addition of Cl2 to C = C occurs in a similar way.

CH3 – CH = CH2 Reactions of alkenes H H HI C C I ADDITION of Hydrogen Halides (HCl, HBr, HI) H C H C I HI Carried out in gas state or in non-polar solvents What happens if an unsymmetrical alkene is used e.g. propene? CH3 – CH = CH2

CH3 – CH = CH2 HBr CH3 - CH – CH3 CH3 – CH2 - CH2 - Br Br Reactions of alkenes CH3 – CH = CH2 HBr CH3 - CH – CH3 Br CH3 – CH2 - CH2 - Br Minor product Major product

RULE for addition of HX across C = C Reactions of alkenes RULE for addition of HX across C = C Markovnikov’s Rule When HX is added across C = C the hydrogen atom attaches to the carbon atom that is already bonded to the greater number of hydrogen atoms. This is thought to involve the attack on the electron rich C = C by an electrophile. H – X bond is permanently polarised.

Mechanism for addition of HX Reactions of alkenes Mechanism for addition of HX C H + C H X δ+ δ- X- Uses π electrons C Rate determining step – produces X- and carbonium ion Carbonium ion is attacked by X- - a nucleophile

Mechanism for addition of HX Reactions of alkenes Mechanism for addition of HX C H X C H + X-

Addition of HBr to propene 2 possible carbocation intermediates Reactions of alkenes Addition of HBr to propene 2 possible carbocation intermediates 1 - C - C -- C H + Electron donating effect 2 H C +

Addition of HBr to propene Reactions of alkenes Addition of HBr to propene H C + 2 1 - C - C -- C H + Alkyl groups donate electrons to adjacent atoms Alkyl groups can help stabilise the positive charge in these carbocations. Which carbocation is most stable? Why? Two alkyl groups are present to help stabilise the positive charge This carbocation is formed in preference

Addition of HBr to propene Reactions of alkenes Addition of HBr to propene Most likely product H C + Preferred carbocation H C H C Br C H 2-bromopropane

Reactions of alkenes HI + H2O  H3O+(aq) + I-(aq) Acid catalysed addition of water HI + ethene in an aqueous solution Strong acid HI + H2O  H3O+(aq) + I-(aq) dissociates Hydronium or oxonium ion Attacks double bond! electrophile or nucleophile electrophile or nucleophile

Mechanism for addition of WATER Reactions of alkenes Mechanism for addition of WATER C H + O H C H O - H+ C H O + C H O

Reactions of alkenes C H SO3H O C H S O H H2O H H C H H2SO4 H C O H H Another Mechanism for the addition of WATER Cold conc. Sulphuric acid or phosphoric acid C H SO3H O C H Ethyl hydrogen sulphate S O H H2O H Hydrolysed by warming with water H C H H2SO4 H C O H H

Reactions of alkenes Addition of water across a double bond follows Markovnikov’s rule. Good method for synthesising specific alcohols.

Reactions of alkenes alkanes halogenoalkanes alkene impurities alkanes H2SO4 method of adding water is used to purify substances contaminated with alkenes. alkanes halogenoalkanes alkene impurities Alkanes and halogenoalkanes are insoluble in conc. sulphuric acid bubbling through or shaking with sulphuric acid alkanes halogenoalkanes alcohol Alcohol dissolves in the acid. Alkane or halogenoalkane is then obtained by distillation

Halogenoalkanes Rare in the natural world Synthesised in the lab Widely used in medicine, manufacture of plastics and agriculture 1847 CHCl3 (chloroform) – general anaesthetic first used by James Simpson (Bathgate) Problem – overuse of pesticide and implicated in damage to ozone layer and

Naming Halogenoalkanes Halogenoalkanes (also haloalkanes and alkylhalides) Halogen atom  prefixes CHLORO- BROMO-, FLUORO-, IODO- More than one halogen atom of a type  prefixes mono-, di-, tri-, tetra- Basic name  longest unbranched chain Position of halogen  number in front of prefix

Naming Halogenoalkanes C H Cl numbering 2,3-dichloropentane

Naming Halogenoalkanes C H Br numbering 3-bromobut-1-ene

Classification of Halogenoalkanes monohalogenoalkanes primary 1o secondary 2o tertiary 3o C X R H C X H R C X R

Naming Halogenoalkanes C H Br Cl numbering 2,3-dibromo-1-chloro-2-methylbutane

They are immiscible in water. Halogenoalkanes Physical Properties The carbon-halogen bond is polar but this doesn’t have a great influence on physical properties. They are immiscible in water. The boiling point depends on the size of the molecule and the halogen. Compounds with Cl or Br have higher boiling points than those with F.

Reactions of Halogenoalkanes Reactions depend on 2 factors The halogen present  strength of C – X bond R – I > R- Br > R – Cl > R - F Weakest Strongest bond bond Reactive Very unreactive The position of C – X bond in the molecule

Reactions of Halogenoalkanes 1) Nucleophilic substitution 2 main reactions 1) Nucleophilic substitution 2) Elimination

Nucleophilic substitution Reactions Polarised bond Carbon atom is susceptible to nucleophilic attack δ+ δ- C X higher electronegativity

C X δ+ δ- Nucleophilic substitution Reactions Polarised bond higher Carbon atom is susceptible to nucleophilic attack If bond breaks heterolytically an X- ion is formed. Cl-, Br-, I- are all good leaving groups because they are stable. δ+ δ- C X higher electronegativity

Nucleophilic substitution Reactions Nucleophilic substitution Halogens facilitate (make possible) the heterolytic cleavage (breaking) of the bond Nucleophilic substitution C Y C X δ+ δ- + X- Y-

C2H5Br(l) + OH-(aq)  C2H5OH(aq) + Br-(aq) Nucleophilic substitution – 2 mechanisms 1) Hydrolysis of bromoethane (PRIMARY halogenoalkane) Using aqueous alkali C2H5Br(l) + OH-(aq)  C2H5OH(aq) + Br-(aq) This reaction is first order with respect to HYDROXIDE IONS and first order with respect to BROMOETHANE Rate = k[C2H5Br][OH-] RDS involves one mole of each

C Br OH- C Y Br - δ+ δ- transition state Nucleophilic substitution Reactions C Br δ+ δ- OH- This type of reaction is known as an SN2 reaction C Y Br - transition state

(CH3)3CBr(l) + H2O(l)  (CH3)3COH(aq) + HBr(aq) Nucleophilic substitution – 2 mechanisms 2) Hydrolysis of 2-bromo-2 methylpropane (TERTIARY halogenoalkane) Using water (CH3)3CBr(l) + H2O(l)  (CH3)3COH(aq) + HBr(aq) This reaction is first order with respect to HALOGENOALKANE but Zero order with respect to water Rate = k[(CH3)3CBr] RDS only involves the halogenoalkane

- + CH3 H3C CH3 H3C C Br C Br Rate = k[(CH3)3CBr] slow Nucleophilic substitution – 2 mechanisms 2) Hydrolysis of 2-bromo-2 methylpropane (TERTIARY halogenoalkane) using water Rate = k[(CH3)3CBr] RDS only involves the halogenoalkane C Br δ+ δ- CH3 H3C C Br - CH3 H3C + slow heterolytic cleavage planar carbocation

+ CH3 H3C CH3 H3C C C OH + H+ + H2O fast slow Nucleophilic substitution – 2 mechanisms 2) Hydrolysis of 2-bromo-2 methylpropane (TERTIARY halogenoalkane) using water C OH CH3 H3C C CH3 H3C + + H2O fast slow + H+ This type of reaction is known as an SN1 reaction

Order of stability: 1o < 2o < 3o Nucleophilic substitution Mechanism depends (partly) on the type of halogenoalkane Primary and secondary halogenoalkanes: hydrolysed by SN2 mechanism Tertiary halogenoalkanes: hydrolysed by SN1 mechanism. (elimination can take over) This is because stability of the carbocation intermediate is important Order of stability: 1o < 2o < 3o

Nucleophilic substitution Alkyl groups push electrons towards neighbouring carbon atoms. In tertiary carbocations, 3 alkyl groups help stabilise the positive charge. A primary carbocation, with only one alkyl group will be much less stable.

Uses of substitution of halogenoalkanes Reactants: water or aqueous alkali. Products : specific alcohols which can then be converted to aldehydes or ketones. Aldehydes can then be oxidised to alkanoic acids. R - X  R - OH e.g. CH3CH2CH2CH2Cl  CH3CH2CH2CH2OH

Uses of substitution of halogenoalkanes Reactant: ammonia (weakly nucleophilic due to lone pair of electrons on the N atom and the polar nature of N-H bond) Products : amines R - X  R – NH2 e.g. CH3CH2I  CH3CH2NH2

Uses of substitution of halogenoalkanes Reactant: ammonia Products : amines I- N H C CH3 + N H C H CH3 I - H+ N H C CH3

Uses of substitution of halogenoalkanes sodium + dry alcohol  hydrogen + sodium alkoxide Na(s) + C2H5OH(l)  H2(g) + C2H5O- Na+ The alkoxide ion is a powerful base and a powerful nucleophile Reactant: sodium alkoxide Products : ethers sodium ethoxide + bromomethane  methoxyethane + sodium bromide C2H5O- Na+ + BrCH3  C2H5OCH3 + Na + Br - This reaction is favoured at lower temperature

Uses of substitution of halogenoalkanes Reactant: cyanide ion (CN-) [potassium cyanide in ethanol/heated under reflux] Products : nitriles CH3CH2CH2I + CN-  CH3CH2CH2CN + I- 1-iodo propane butanenitrile Main advantage: The carbon chain has been increased by one carbon Nitriles can be converted into the corresponding acid by acid hydrolysis or into an amine by reduction using lithium aluminium hydride.(LiAlH4)

Uses of substitution of halogenoalkanes CH3CH2CH2 - I  CN- CH3CH2CH2 - C N [nitrile] Acid hydrolysis LiAlH4 CH3CH2CH2 C O OH CH3CH2CH2 CH2 - NH2 [amine]

Products of substitution alcohols ethers amines nitriles R-X reagents NH3 OH- RO- in alcohol CN- in alcohol

Elimination Reactions Reactant: strong base [eliminates hydrogen halide molecule] Product : alkene Good nucleophiles are often bases – elimination and substitution can take place at the same time The reaction conditions determine which process dominates. In water [polar solvent] substitution predominates. In ethanol it’s elimination.

Elimination v substitution CH3CH2CH2CH2 - Br 1-bromobutane KOH in aqueous solution KOH in ethanol solution CH3CH2CH2CH2 - OH CH3CH2CH CH2 butan-1-ol but-1-ene There are two possible mechanisms for elimination reaction

- Elimination H Br C R’’ R’ δ+ δ- HO H C R’’ R’ - H 2 O Br- One step process: removal of H+ ion and loss of halide ion occur simultaneously CH3CH2CH2CH2 - Br 1-bromobutane KOH in ethanol solution H Br C R’’ R’ δ+ δ- CH3CH2CH =CH2 but-1-ene HO - H C R’’ R’ - H 2 O This mechanism is known as an E2 reaction [Elimination , TWO particles involved in RDS] Br-

E1 reaction - Elimination R’ R’’ H C Br R δ+ δ- R’ R’’ H C R + + Br - CH3CH2CH2CH2 - Br 1-bromobutane + Br - slow KOH in ethanol solution CH3CH2CH =CH2 R’ R’’ H C R + but-1-ene This mechanism is known as an E1 reaction [Elimination , ONE particle involved in RDS] Two step process: 1) C-X bond breaks heterolytically to form a carbocation. 2) Hydrogen ion is removed from adjacent carbon atom forming the C=C. R’ R’’ H C R fast + H2O HO -

Try these questions Name and draw the products for the following reactions and indicate the product that will be formed in the higher yield. The reaction of propene with hydrogen bromide The reaction of 2-methylbut-2-ene with hydrogen chloride The catalytic hydration of 2-methylbut-1-ene The addition of hydrogen bromide to 3-methylbut-1-ene The answers are shown on the following slides

Answers a) The reaction of propene with hydrogen bromide Names of products structures Higher yield Higher yield H C propene H C Br H C Br 2-bromopropane 1-bromopropane

Answers b) The reaction of 2-methylbut-2-ene with hydrogen chloride Names of products structures Higher yield 2-methylbut-2-ene H C CH3 H Higher yield H C CH3 Cl H C CH3 Cl 2-chloro-2-methylbutane 3-chloro-2-methylbutane

Answers c) The catalytic hydration of 2-methyl-but-1-ene Names of products structures Higher yield 2-methylbut-1-ene H C CH3 H Higher yield H C OH CH3 H C CH3 OH 2-methylbutan-1-ol 2-methylbutan-2-ol

Answers d) The addition of hydrogen bromide to 3-methylbut-1-ene structures Names of products Higher yield 3-methylbut-1-ene H C CH3 Higher yield H C CH3 Br H C CH3 Br 4-bromo-2-methylbutane 3-bromo-2-methylbutane

More questions Draw structures for the possible products when 2-chloro-2-methylbutane is reacted with: aqueous potassium hydroxide a solution of potassium hydroxide in ethanol a solution of sodium ethoxide in ethanol? Structure of 2-chloro-2-methylbutane H C Cl CH3 Answer to a)

Answer a) Possible product when 2-chloro-2-methylbutane is reacted with: aqueous potassium hydroxide Substitution Nucleophilic Alcohol produced H C Cl CH3 H C OH CH3 2-chloro-2-methylbutane 2-methylbutan-2-ol Answer to b)

Elimination predominant Answer b) Possible product when 2-chloro-2-methylbutane is reacted with: b) a solution of potassium hydroxide in ethanol Elimination predominant Alkene produced H C Cl CH3 H C CH3 2-chloro-2-methylbutane 2-methylbut-1-ene H C CH3 Answer c) 2-methylbut-2-ene

Answer c) Possible product when 2-chloro-2-methylbutane is reacted with: b) a solution of sodium ethoxide in ethanol H C Cl CH3 Substitution Ether produced H C O CH3 C2H5 2-chloro-2-methylbutane