Haloalkanes and Haloarenes

Haloalkanes (Halogenoalkanes)

• The structure contains the functional group C-X, where X represents a halogen such as Fluorine, Chlorine, Bromine, or Iodine.

Types

• Haloalkanes are compounds where a halogen is bonded to an aliphatic skeleton, forming an alkyl group
• Haloarenes feature a halogen directly bonded to a benzene (aromatic) ring.

Classification

• Compounds are classified by what is directly attached to the functional group.
• A methyl group has the structure (CH_3).
• Primary haloalkanes (1^\circ) have one carbon attached to the carbon with the halogen.
• Secondary haloalkanes (2^\circ) have two carbons attached to the carbon with the halogen.
• Tertiary haloalkanes (3^\circ) have three carbons attached to the carbon with the halogen.

Nomenclature

• Naming includes a prefix that indicates the halogens and their position on the parent alkane chain
• 1-chloropropane has the formula (CH_3CH_2CH_2Cl).
• 2-chloropropane has the formula (CH_3CHClCH_3).
• 1,2-dichloropropane has the formula (CH_2ClCHClCH_3).
• 2-bromo-2-methylpropane has the formula (CH_3CBr(CH_3)CH_3).

Physical Properties

• Boiling points increase with mass
• For isomeric compounds, greater branching lowers the boiling point.
• Haloalkanes are soluble in organic solvents but insoluble in water because they are not polar enough and cannot form hydrogen bonds

Nucleophilic Substitution Reactions

• Halogens exhibit greater electronegativity than carbon, resulting in a dipole within the C-X bond, which becomes polar
• The carbon in the haloalkane is prone to nucleophilic attack because of the dipole.

Nucleophiles

• Examples include (OH^-), (CN^-), (NH_3), and (H_2O).
• Nucleophiles possess at least one lone pair of electrons
• Nucleophiles are attracted to the slightly positive (electron deficient) carbon atom in the haloalkane

Reaction Mechanism

• The nucleophile uses its lone pair to provide electrons for a new bond with the carbon
• Since carbon can only accommodate 8 electrons in its outer shell, a halide ion is displaced, which is substitution
• The reaction is known as nucleophilic substitution.

Reaction Rate

• The rate of reaction depends on the strength of the C-X bond, not its polarity
• The bond dissociation energy for C-I is 238 kJ/mol, making it the weakest bond and easiest to break
• The bond dissociation energy for C-Br is 276 kJ/mol.
• The bond dissociation energy for C-Cl is 338 kJ/mol.
• The bond dissociation energy for C-F is 484 kJ/mol, making it the strongest bond

Practical Investigation

• The silver halide precipitate for AgCl is white
• The silver halide precipitate for AgBr is cream
• The silver halide precipitate for AgI is yellow
• AgF is soluble.

Reactions with NaOH

• The reagent is aqueous sodium or potassium hydroxide
• The conditions involve reflux in an aqueous solution (the solvent is critical)
• The product is an alcohol.
• The nucleophile is the hydroxide ion ((OH^-)).
• For example: (C_2H_5Br(l) + NaOH(aq) \rightarrow C_2H_5OH(l) + NaBr(aq))

Reaction Mechanism

• The hydroxide ion ((OH^-)) acts as a nucleophile, attacking the carbon and displacing the bromide ion
• Important to specify the solvent when answering questions as elimination reactions occur when ethanol is the solvent

Hydrolysis

• The reaction with water is slower

Hydrolysis Reasons

• Liquids are immiscible, resulting in less chance of molecules colliding
• Water is a poor nucleophile.
• The reaction is faster in an alcohol/water mixture because the liquids are miscible

Reaction with (AgNO_3)

• Used to identify the halide in a haloalkane

Reaction with KCN

• The reagent used is aqueous, alcoholic potassium (or sodium) cyanide.
• The reaction conditions are reflux in an aqueous, alcoholic solution.
• The product of the reaction is a nitrile (cyanide).
• The nucleophile in the reaction is the cyanide ion ((CN^-)).
• A typical equation for the reaction: (C_2H_5Br + KCN(aq/alc) \rightarrow C_2H_5CN + KBr)

Mechanism

• The (CN^-) group extends the carbon chain by one carbon atom
• The (CN) group can then be converted to carboxylic acids or amines
• The hydrolysis of (C_2H_5CN) yields (C_2H_5COOH + NH_3)
• The reduction of (C_2H_5CN) yields (C_2H_5CH_2NH_2)

Reaction with (NH_3)

• The reagent is aqueous, alcoholic ammonia (in excess)
• The reaction conditions involve reflux in an aqueous, alcoholic solution under pressure
• The product is an amine (or its salt due to a reaction with the acid produced)
• The nucleophile is ammonia ((NH_3))
• Reaction Example: (C_2H_5Br + NH_3 (aq/alc) \rightarrow C_2H_5NH_2 + HBr)

Using Excess Ammonia

• The second ammonia molecule ensures the removal of HBr, preventing the formation of a salt
• A large excess of ammonia prevents further substitution

Other Reactions of Haloalkanes: Friedel-Crafts Alkylation

• It substitutes an alkyl group (e.g. methyl, ethyl) onto a benzene ring
• The reagents used are a haloalkane (RX) and anhydrous aluminum chloride ((AlCl_3))
• The reaction occurs at room temperature in a dry, inert solvent (ether), which is an electrophilic substitution
• The electrophile is a carbocation ion (R^+) (e.g., (CH_3^+))

Preparation of Haloalkanes

• From alkanes, using (CH_4 + Cl_2 \rightarrow CH_3Cl + HCl) through free radical substitution under UV light.
• From alkenes, using (C_2H_4 + HBr \rightarrow C_2H_5Br) through electrophilic addition with no catalyst or UV light.
• From alcohols, using (C_2H_5OH + HBr \rightarrow C_2H_5Br + H_2O) through protonation of alcohol with an acid catalyst.

Uses of Haloalkanes

• They are important in synthetic organic chemistry due to the reactivity of the C-X bond, which allows the halogen to be replaced by different groups via nucleophilic substitution
• Substitution of a bromine atom occurs during the manufacture of ibuprofen
• Chloroethene ((CH_2=CHCl)) to poly(chloroethene), or PVC, with the formula ( -(CH_2-CHCl)_n-), used for packaging
• Tetrafluoroethene ((CF_2=CF_2)) to poly(tetrafluoroethene), or PTFE, with the formula (-(CF_2-CF_2)_n-), which is used in non-stick surfaces

CFCs

• Dichlorofluoromethane ((CHFCl_2)) used as a refrigerant
• Trichlorofluoromethane ((CF_3Cl)) used as an aerosol propellant
• Bromochlorodifluoromethane ((CBrClF_2)) used in fire extinguishers
• (CCl_2FCC/F_2) used as a dry cleaning solvent
• All of the compounds were chosen because of their low reactivity, volatility, and non-toxicity

Problems with CFCs

• CFCs have been blamed for environmental thinning of the ozone layer, harming the earth
• Ozone absorbs a lot of harmful UV radiation
• CFCs break up in the atmosphere to form free radicals like: (CF_2Cl_2 \rightarrow \cdot CF_2Cl + Cl)
• The carbon radicals then catalyze ozone decomposition

The Decomposition

• (Cl+O_3 \rightarrow \cdot ClO + O_2)
• (ClO + O \rightarrow Cl + O_2)
• Overall (2O_3 \rightarrow 3O_2)

Solution

• CFC's were designed by chemists to help people
• chemists now synthesise alternatives to CFC's to protect the environment such as hydrocarbons and HCFC's
• (CO_2) can be use as an alternative blowing agent
• This will allow the reversal of the ozone layer problem

Elimination Reactions of Haloalkanes

• The solvent influences the products of reactions between haloalkanes and (OH^-), with the choice of solvent favoring one route

Solvent & Product Table

Elimination Mechanism

• The hydroxide ion acts as a base and picks up a proton
• The proton comes from a carbon atom next to the one bonded to the halogen
• The electron pair left moves to form a second bond between the carbon atoms
• The halide ion is displaced
• There is elimination of HBr

Haloalkane Unsymmetrical

• If the haloalkane is unsymmetrical (e.g., 2-bromobutane), a mixture of isomeric alkene products is obtained