Advanced Higher Chemistry Unit 3 Halogenoalkanes

1. Advanced Higher Chemistry Unit 3 Halogenoalkanes

2. Halogenalkanes • Also known as haloalkanes or alkyl halides • Organic compounds containing halogens are rare in the natural world i.e. most are synthetic. • Important uses include • in medicine, e.g. chloroform (trichloromethane) • in agriculture as pesticides • in plastics e.g. PVC, PTFE • as solvents e.g. carbon tetrachloride • Unfortunately they are implicated in environmental damage to the planet, notably in the overuse of pesticides and in damage to the ozone layer.

3. H R C X H R R C X H R R C X R Classification of halogenoalkanes • Classified as primary (1°) secondary (2°) tertiary (3°) Primary – the carbon atom carrying the halogen has only one alkyl group or two hydrogen atoms attached to it. Secondary – the carbon atom carrying the halogen has two alkyl groups or one hydrogen atom attached to it. Tertiary – the carbon atom carrying the halogen has three alkyl groups or no hydrogen atoms attached to it.

4. Nomenclature of halogenoalkanes • The more complicated the molecule, the greater the possibility there is for structural isomerism. • The presence of a halogen atom is shown by the appropriate prefix: fluoro-, chloro-, bromo-, or iodo-. • If the molecule contains more than one halogen atom of the same type this is shown by the prefixes di-, tri-, tetra-, etc. • The position of the halogen is shown by a number in front of the prefix. • The substituents are listed in alphabetical order e.g. dibromo comes before chloro because ‘b’ comes before ‘c’. Prefixes such as di and tri are ignored.

5. Examples 2,3-dichloro-3-methylpentane 1,2-dichloropropane 3-bromo-2-methylpentane

6. Exercise • Now complete the exercise on page 18 of your unit 3(b) notes.

7. Bonding in Halogenalkanes • All bonds in halogenalkanes are sigma bonds (see Bonding in Alkanes). Synthesis of Halogenalkanes • See Alkenes – Hydrogen halide addition for monohalogenalkanes. • See ‘Alkenes – Halogenation’ for dihalogenalkanes.

8. Reactions of Halogenalkanes • Two main types of reaction • Nucleophilic Substitution • Elimination • The C-X bond is fairly polar, due to the difference in electronegativity between carbon and the halogens. • Reactivity seems to be related to the bond strength since the order of reactivity is generally R-I > R-Br > R-Cl > R-F weakest strongest bond bond and to the position of the carbon-halogen bond within the molecule.

9. Halogenalkanes - Nucleophilic Substitution • The following terms are often used when discussing substitution reactions – Y- + R3C-X  R3C-Y + X- LEAVING GROUP NUCLEOPHILE SUBSTRATE PRODUCT • Because the C-X bond is polar, with the C carrying a partial positive charge, the C will be susceptible to attack by nucleophiles. • If the C-X bond breaks heterolytically, and X- ion will be formed. Cl-, Br- and I- are all stable ions and are regarded as good leaving groups i.e. the presence of these atoms in a molecule will facilitate the heterolytic cleavage of a bond.

10. Experimental evidence has shown that there are two possible mechanisms for nucleophilic substitution reactions • The SN2Reaction • The SN1 Reaction

11. Halogenalkanes - The SN2 Reaction E.g. – Hydrolysis of bromoethane, a primary halogenalkane, in an aqueous alkali solution. C2H5Br(l) + OH-(aq)  C2H5OH(aq) + Br-(aq) • A study of the reaction kinetics show that the reaction is first order with respect to (w.r.t.) hydroxide ions and first order w.r.t. bromoethane. i.e. Rate = k[C2H5Br][OH-] (see Unit 2) • This means the Rate Determining Step (RDS) must involve a bromoethane molecule and a hydroxide ion

12. SN2 – The Mechanism Transition State • The nucleophilic hydroxide ion approaches the partial positive carbon (from the opposite side of the bromine atom). • A bond begins to form between the oxygen and carbon atoms, at the SAME time the carbon-bromine bond weakens. • A transition state will form with a ½ O-C bond and ½ C-Br bond, only IF there was enough energy in the collision. • The O-C bond forms completely, the C-Br bond breaks completely NB - If the initial halogenalkane is chiral (see later) this causes an inversion of chirality. For this reason the 3-D representation of this mechanism IS IMPORTANT!!

13. S Substitution Nucleophilic N 2 RDS involves 2 particles

14. Halogenalkanes - The SN1 Reaction • E.g. – Hydrolysis of 2-bromo-2-methylpropane, a tertiary halogenalkane, in water. (CH3)3CBr(l) + H2O(l)  (CH3)3COH(aq) + HBr(aq) • A study of the reaction kinetics show that the reaction is first order w.r.t. the halogenoalkane but zero order w.r.t. water. i.e. Rate = k[(CH3)3CBr(l) ] • This means the Rate Determining Step (RDS) must involve only the halogenalkane.

15. SN1 – The Mechanism

16. The C-Br bond breaks heterolytically forming a planar carbocation (stabilised by the electron donating effect of the alkyl groups, see later slide) and a bromide ion. • The nucleophilic O atom on the water can then attack the +ve carbon atom and form the alcohol. NOTE If the halogenalkane is chiral (see later), the product will be a racemic mixture (see later) as the intermediate carbocation is planar and can be attacked from either side. For this reason the 3-D representation of this mechanism is NOT important!!

17. S Substitution Nucleophilic N RDS involves 1 particle 1

18. SN1 or SN2 Hydrolysis? • SN1 favoured by – • Tertiary halogenalkanes (carbocation stabilised by alkyl groups) • Highly polar solvents • SN2 favoured by – • Primary and secondary halogenalkanes • Presence of OH-ions (i.e. alkaline solution)

19. Stability of carbocations • The order of stability of carbocations is: primary < secondary < tertiary • Alkyl groups have a tendency to push electrons towards a neighbouring carbon atom hence, in a tertiary carbocation the three alkyl groups help stabilise the positive charge on the tertiary carbon atom. A primary carbocation has only one alkyl group so will therefore be much less stable.

20. Halogenalkanes – Importance of Substitution • Synthesis of • Specific Alcohols (hence ketones, aldehydes and alkanoic acids) • Amines (using ammonia) • Synthesis of ethers • Synthesis of nitriles

21. Halogenalkanes – Synthesis of Alcohols R-X  R-OH • Alcohols can then be oxidised to aldehydes or ketones. Aldehydes can then be oxidised to form alkanoic acids. • See SN1 and SN2 mechanism for specific examples.

22. Halogenalkanes – Synthesis of Amines Alkylammonium ion (intermediate) • The polarity of the N-H bond and the lone pair of electrons allow ammonia to act as a nucleophile. • The ammonia molecule attacks the slightly positive carbon atom, displacing the halide ion. Removal of the hydrogen ion then produces the amine.

23. Halogenalkanes – Synthesis of Ethers • Especially for unsymmetrical ethers. e.g. C2H5O- Na+ + BrCH3 C2H5OCH3 + Na+ Br- • Reaction is carried out at low temperature, otherwise elimination reaction may dominate (due to the alkoxide ion being a base as well as a nucleophile) NOTEAll nucleophiles are bases and vice versa • Sodium ethoxide is produced by the reaction of sodium with a dry sample of alcohol. e.g. Na+ C2H5OH  C2H5O- Na+ +H2

24. Halogenalkanes – Synthesis of Nitriles e.g. CH3CH2CH2I + K+CN-  CH3CH2CH2CN + I- • CN- is the cyanide ion. • Reaction is carried out under reflux. • Reaction is useful as it extends the carbon chain. • Nitriles can then be converted into alkanoicacids or amines.

25. Halogenalkanes – Elimination Reaction • Halogenalkanes will form alkenes in the presence of a strong base. • This involves the removal (i.e. elimination) of a hydrogen halide. e.g. CH3CH2CH2Br  CH3CH=CH2 + HBr • Nucleophiles are bases and vice versa, so in a reaction there will be elimination and substitution reactions occurring at the same time. • The reaction conditions will determine which process dominates.

26. There are two possible mechanisms, E1 and E2. • The mechanism that dominates will depend on the strength of the base and the environment of the halogen atom (1°, 2° or 3°)

27. Halogenalkanes – E1 Reaction • Favoured for tertiary halogenalkanes due to stabilisation of the carbocation by the electron donating effect of the alkyl groups. • Only 1 particle involved in the RDS.

28. Halogenalkanes – E2 Reaction • 2 particles involved in the RDS.

29. Exercise • Complete the exercise on page 24 of your Unit 3(b) notes.