Reactions of aloalkanes N Goalby chemrevise.org aloalkanes ontain a halogen atom covalently bonded to a carbon atom General formula: R-X where X is a halogen atom (F, l,, I) and R is the carbon chain. Most halogens are more electronegative than carbon and so a dipole is induced and the -X bond is polar. δ+ F δ- > δ+ l δ- > δ+ δ- 1
Naming haloalkanes Based on original alkane name, with a prefix indicating halogen atom: Fluoro for F; hloro for l; omo for ; Iodo for I. Substituents are listed alphabetically. 1-bromopropane l 2-chloro-2-methylbutane 2-bromopropane l l There are two chlorine atoms attached: dichloro l 3-bromo-1-chloropentane 1,2-dichloropropane lassifying haloalkanes aloalkanes can be classified according to how many carbons are attached to the -X functional group. Primary aloalkane 1 o One carbon attached to the carbon atom adjoining the halogen Secondary aloalkane 2 o Two carbons attached to the carbon atom adjoining the halogen l Tertiary aloalkane 3 o Three carbons attached to the carbon atom adjoining the halogen 2
Nucleophilic Substitution Reactions of aloalkanes Nucleophile: electron pair donator e.g. :O -, :N 3, N - Substitution: swapping a halogen atom for another atom or groups of atoms The ATTAKING GROUP is a NULEOPILE (e.g. O -, N -, 2 O, N 3 ) attracted to the δ+ carbon and donating a lone pair of electrons to form a new covalent bond Nu: - δ + X δ - Nu + X - The carbon has a small positive charge because of the electronegativity difference between the carbon and the halogen A curly arrow will always start from a lone pair of electrons or the centre of a bond Reactivity of haloalkanes The rate of reaction depends on the strength of the -X bond. The weaker the bond, the easier it is to break and the faster the reaction. The iodoalkanes are the fastest to substitute and the fluoroalkanes are the slowest. The strength of the -F bond is such that fluoroalkanes are very unreactive Bond enthalpy / kjmol -1 -I 238-276 -l 338 -F 484 3
omparing the rate of hydrolysis reactions ydrolysis is defined as the splitting of a molecule ( in this case a haloalkane) by a reaction with water Water is a poor nucleophile but it can react slowly with haloalkanes in a substitution reaction 3 2 X + 2 O 3 2 O + X - + + Aqueous silver nitrate is added to a haloalkane and the halide leaving group combines with a silver ion to form a SILVER ALIDE PREIPITATE. The precipitate only forms when the halide ion has left the haloalkane and so the rate of formation of the precipitate can be used to compare the reactivity of the different haloalkanes. The iodoalkane forms a precipitate with the silver nitrate first as the -I bond is weakest and so it hydrolyses the quickest 3 2 I + 2 O 3 2 O + I - + + Ag + (aq) +I - (aq) AgI (s) yellow precipitate The quicker the precipitate is formed, the faster the substitution reaction and the more reactive the haloalkane AgI (s) - yellow precipitate Ag (s) cream precipitate Agl (s) white precipitate forms faster Reaction of haloalkanes with aqueous hydroxide ions hange in functional group: haloalkane alcohol Reagent: potassium (or sodium) hydroxide onditions: In aqueous solution; eat under reflux Mechanism: Nucleophilic Substitution Type of reagent: Nucleophile, O - Equation: 3 2 2 (l) + K + O - (aq) 3 2 2 O (l) + K + - (aq) + KO O + K 1-bromopropane propan-1-ol 4
Nucleophilicsubstitution mechanism, S N 2 omoethane [Primary haloalkane] + aqueous hydroxide ions 3 δ+ δ- 3 O 3 - O - The attacking group is a NULEOPILE, O -. It is attracted to the slightly positive carbon atom and has a lone pair of electrons which can form a new covalent bond. This mechanism goes through a TRANSITION STATE: the -O bond is forming at the same time as the - bond is breaking. O EXTRA Nucleophilic substitution mechanism omoethane [Primary haloalkane] + aqueous hydroxide ions δ+ δ- 3 3 - O ethanol O - The attacking group is a NULEOPILE, O -. It is attracted to the slightly positive carbon atom and has a lone pair of electrons which can form a new covalent bond. Overall, there has been SUBSTITUTION of a atom for a O group. 5
Nucleophilicsubstitution mechanism, S N 1 2-methyl-2-bromobutane [Tertiary haloalkane] + aqueous hydroxide ions 3 3 δ+ δ- 3 3 + - 3 3 O - 2 3 2 3 - O 2 3 2-methylbutan-2-ol An intermediate ARBOATION is formed. The O - NULEOPILE attacks the positively charged carbon, using its lone pair to form a covalent bond. Overall, there has been SUBSTITUTION of a atom for a O group. EXTRA S N 1 or S N 2? The form of nucleophilic substitution known as S N 2 is a bimolecular process; in the initial step there is collision between the haloalkane and the nucleophile The alternative mechanism, S N 1, involves the initial breaking of the -X bond to form a carbocation), which is then attacked by the nucleophile. S N 1 is favoured for tertiary haloalkanes where there is steric hindrance to attack (lots of bulky alkyl groups get in the way of the incoming nucleophile) and a more stable tertiary carbocation intermediate is formed (the alkyl groups are electron releasing and reduce the charge density on the carbon atom). EXTRA 6
Reaction of haloalkanes with ammonia hange in functional group: haloalkane amine Reagent: N 3 dissolved in ethanol onditions: eating under pressure (in a sealed tube) Mechanism: Nucleophilic Substitution Type of reagent: Nucleophile, :N 3 Equation: 3 2 2 (l) + 2N 3 (alc) 3 2 2 N 2 (l) + N 4 (aq) + 2N 3 N 2 + N 4 1-bromopropane propylamine Nucleophilic substitution mechanism For formation of primary amine from haloalkane δ+ δ- l - 3 l 3 N + N 3 N 3 This reaction does not give a good yield and various other side products are formed. 3 N 2 N 4+ l - 7
Reaction of haloalkanes with KN hange in functional group: haloalkane nitrile Reagent: KN dissolved in ethanol onditions: eating under reflux Mechanism: Nucleophilic Substitution Type of reagent: Nucleophile, :N - : N - Equation: 3 2 2 + N - 3 2 2 N + - + :N - N + - 1-bromopropane butanenitrile Nucleophilic substitution mechanism cyanide ion with bromoethane 3 N - δ+ δ- 3 N propanenitrile - The in the N - ion counts as part of the carbon chain so remember to add one carbon to the name. E.g. omoethane turns into propanenitrile 8
Reaction of haloalkanes with alcoholic hydroxide ions hange in functional group: haloalkane alkene Reagents: Potassium (or sodium) hydroxide onditions: In ethanolic solution; eat Mechanism: Elimination Type of reagent: Base, O - Equation: 3 2 2 (l) + K + O - (alc) 3 2 (g) + K + - + 2 O + KO + K + 2 O 1-bromopropane propene Elimination mechanism 2-bromopropane + alcoholic hydroxide ions 3 O - 3 - O The hydroxide ion, O -, acts as a BASE and removes a proton (hydrogen ion). The proton comes from a carbon atom adjacent to the one bonded to the bromine. 9
Elimination from unsymmetrical haloalkanes What alkenes could this form when reacted with alcoholic KO? l The O - removes a proton from a carbon atom adjacent to the carbon bearing the halogen. 2-chloro-2-methylbutane is unsymmetrical and there are two adjacent carbon atoms. ydrogen can be removed from either and a mixture of isomeric alkenes is formed. Apparatus used for Elimination Reaction eramic wool soaked in KO in ethanol and halogenoalkane Gaseous alkene EAT EXTRA 10
Elimination or substitution? The products of reactions between haloalkanes and O - are influenced by the solvent that the NaO is dissolved in. Both reactions can take place simultaneously and a mixture of alkenes and alcohols may be formed. owever, the choice of solvent can favour one reaction. Solvent Product Role of O - Mechanism Water Alcohol Nucleophile Nucleophilic substitution Alcohol Alkene Base Elimination 11