Alkyl Halides: Classification and Nomenclature

Alkyl Halides

• Alkyl halides are known as haloalkanes or halogenoalkanes.
• Alkyl halides contain one or more halogen atoms covalently bonded to sp3 hybridized carbon atoms of an alkyl group.
• The general formula for alkyl halides is CnH2n+1X, where X = F, Cl, Br, or I.

Classification of Alkyl Halides

• Alkyl halides are classified based on the carbon atom bearing and degree of substitution of the halogen.
• Primary alkyl halides (1°) have the halogen-bearing carbon attached to one other alkyl group or none.
• Secondary alkyl halides (2°) have the carbon with the halogen attached joined directly to two other alkyl groups, which may be the same or different.
• Tertiary alkyl halides (3°) have the halogen attached directly to three alkyl groups, which may be any combination of the same or different.

Nomenclature of Alkyl Halides

• Names are derived from the parent alkane.
• Halogens are named as prefixes:
  • Fluorine (F) becomes Fluoro-
  • Chlorine (Cl) becomes Chloro-
  • Bromine (Br) becomes Bromo-
  • Iodine (I) becomes Iodo-
  • Astatine (At) becomes Astato-
• The positions of halogen atoms are indicated by numbering the carbon atoms linked to the halogen, using the lowest number possible.
• For multiple identical halogen atoms, prefixes such as di-, tri-, and tetra- are used and listed alphabetically with other substituents.
• If multiple substituents are equidistant from both ends, the name is based on alphabetical order.

Structural Isomerism

• Alkyl halides exhibit structural isomerism, including chain isomers and position isomers.
• Chain isomers involve different arrangements of the alkyl chain.
• Position isomers involve different positions of halogen atoms.

Preparation of Alkyl Halides

• Alkyl halides can be prepared by addition reaction of alkenes with hydrogen halides or halogens.
• Alkyl halides can be prepared by substitution reaction of alcohol with halides.

Reactivity of Alkyl Halides

• Alkyl halides are relatively reactive compounds due to the polar carbon-halogen bond.
• The reactivity depends on the nature of the halogen and the class of alkyl halides.
• The electron deficient carbon atom carrying the halogen is readily attacked by nucleophiles.
• Bond length and bond strength influence the reactivity of alkyl halides.
• Longer bond lengths result in weaker bonds and increased reaction rates.
• Reactivity decreases in the order: iodoalkanes > bromoalkanes > chloroalkanes > fluoroalkanes.
• The relative stability of carbocation ions increases in the order: primary < secondary < tertiary.
• Reactivity of alkyl halides with the same halogen in hydrolysis reactions increases in the order: tertiary < secondary < primary.

Reaction of Alkyl Halides

• The two main types of reactions are substitution and elimination.
• In substitution, the halogen atom is replaced by a nucleophile.
• In elimination, the halogen atom is removed from the organic molecule.

Nucleophilic Substitution Reaction

• The general equation: R-X + Nu- → R-Nu + X-
• Common types include:
• Hydrolysis of alkyl halides.
• Formation of amines.
• Formation of nitrile.

Hydrolysis of Alkyl Halides

• Hydrolysis is carried out by boiling alkyl halides with aqueous sodium hydroxide under reflux.
• Hydroxide ions (OH-) act as nucleophiles.
• The halogen atom in alkyl halide is replaced by an -OH group to give an alcohol.
• Primary alkyl halides react in a single-step mechanism.
• The OH- nucleophile donates its electron pair to the electron-deficient C+ of the bromoalkane, forming a single covalent bond.
• The C-Br bond is broken via heterolytic fission, forming a :Br- ion.

Formation of Amines

• Alkyl halide is heated with concentrated ammonia in ethanol in a sealed tube, forming an amine.
• The reaction occurs in two steps:
• Formation of ethylammonium bromide.
• Ammonia removes H+ from ethylammonium ion, producing ethylamine.
• Further reactions of ethylamine occur because ethylamine has a lone pair of electrons (a nucleophile) and can react further with bromoethane.
• The reaction continues until there are no lone electrons on the nitrogen atom.

Formation of Nitriles

• Alkyl halides are heated under reflux with potassium cyanide in ethanol.
• Halogen is replaced by the nucleophile nitrile (-CN) group.
• The number of carbon atoms in the product is increased by one.

Elimination Reaction

• Alkyl halides undergo elimination reactions when heated (78°C) under reflux with anhydrous sodium/potassium hydroxide (dissolved in ethanol).
• Hydrogen and halogen (from adjacent carbon atoms) are eliminated as a proton (H+) and halide (X-) to form a double bond.
• The hydrogen atom is removed from one of the carbon atoms together with bromine from the adjacent carbon atom.
• A double bond is formed between these two carbon atoms, and the OH- ion behaves as a base, accepting a proton H+ to form H2O.
• The reaction of hydroxide ions with alkyl halides yields different products depending on the reaction conditions.
• Under anhydrous conditions, elimination occurs, with OH- acting as a base to produce an alkene.
• Under aqueous conditions, substitution occurs, with OH- acting as a nucleophile to produce an alcohol.

Chemical Tests for Halogen Atoms

• Halogen atoms in alkyl halides can be identified by heating them with a solution of silver nitrate in ethanol.
• A precipitate of the silver halide forms immediately.
• Identification involves the color of the silver halide produced, the time taken for the precipitate to form, and the solubility of silver halide in ammonia solution.
• Results: AgCl is a white solid, AgBr is a pale yellow solid, and AgI is a yellow solid.

Alcohols

• Organic compounds consist of at least one hydroxyl (-OH) functional group.
• The general formula for alcohols is CnH2n+1OH.

Nomenclature of Alcohols

• Step 1: Identify the longest carbon chain joined to the hydroxyl group.
• Step 2: Replace the final 'e' of the corresponding alkane with the suffix 'ol'.
• Step 3: Number the carbon atom so that the position of the hydroxyl group is indicated by the lowest number.
• Step 4: Name all other substituents present.
• Step 5: Write the substituents in alphabetical order before the parent alcohol name.

Classification of Alcohols

• classification depends on the number of alkyl or aryl groups present on the carbon attached to the hydroxyl group.
• Primary (1°) alcohols have one alkyl group attached to the carbon bearing the hydroxyl group.
• Secondary (2°) alcohols have two alkyl groups attached to the carbon bearing the hydroxyl group.
• Tertiary (3°) alcohols have three alkyl groups attached to the carbon bearing the hydroxyl group.

Preparation of Alcohols

• Can be prepared through:
• Indirect hydration of alkenes.
• Hydrolysis of alkyl halides.
• Reaction of Grignard reagents with aldehydes or ketones.

The Indirect Hydration of Alkenes

• An alkene is passed into concentrated sulfuric acid at room temperature or at 80°C to produce alkyl hydrogensulphate.
• The mixture is diluted with water and distilled. Hydrolysis of the alkyl hydrogensulphate produces alcohol.

The Hydrolysis of Alkyl Halides

• When alkyl halide is heated with aqueous sodium hydroxide under reflux, alcohol is produced on hydrolysis.

The Reaction of Grignard Reagents with Aldehydes or Ketones

• Grignard reagents (RMgX) are strong nucleophiles made by dissolving alkylhalides/halobenzenes in dry diethyl ether and allowing them to react with magnesium.
• Example: Reaction of Grignard reagent with methanal (HCHO) involves two steps:
• Addition of the Grignard reagent.
• Hydrolysis in acid.
• Grignard reagents react with:
• Methanal to yield a primary alcohol.
• Other aldehydes to yield secondary alcohols.
• Ketones to yield tertiary alcohols.

Physical Properties of Alcohols

• Higher boiling points are observed when compared to alkanes/alkenes due to the presence of intermolecular hydrogen bonds.
• Alcohols with branched carbon chains have lower boiling points compared to unbranched chain isomers.
• Hydrogen bonds occur between alcohol molecules and water molecules, making alcohols miscible in water.
• Solubility of alcohols in water decreases as the number of carbon atoms increases.
• An alcohol molecule contains a polar end and a non-polar end, rendering it useful as a solvent.
• The polar end is soluble in water, and a non-polar end is soluble in non-polar solvents.

Chemical Properties of Alcohols

• Reactions of the -OH group include:
• Substitution reactions.
• Oxidation reactions.
• Elimination reactions.
• Combustion in air to produce carbon dioxide and water.

Combustion of Alcohols

• Alcohols burn in an excess of oxygen to produce CO2 and H2O (vapor) and release energy.
• Bioethanol is produced by fermentation of plant material and is mixed with petrol and used as motor fuel.
• Bioethanol reduces dependence on crude oil products but at the same time limits the land available for growing food crops.

Reactions of the -OH Group

• Different electronegativities of carbon, oxygen, and hydrogen in alcohols lead to the formation of electron deficient sites; partially positive carbon and hydrogen.
• Alcohols can react by breaking either the C-O or O-H bond, depending on the reagents used.
• Alcohols can undergo two types of reactions which involve the -O-H group:
• Fission (breaking) of the oxygen-hydrogen bond (R-O+H).
• Fission of the carbon-oxygen bond (C+OH).

Fission of the oxygen-hydrogen bond (O-H)

• The Formation of an Alkoxides:
• Alcohols react with sodium to form sodium alkoxides and hydrogen gas.
• Ethanol reacts with sodium to form sodium ethoxide (turns phenolphthalein indicator pink).
• The reactivity of alcohols toward sodium decreases as the acidic strength of the alcohol decreases, following this order: 1° alcohols > 2° alcohols > 3° alcohols.
• Both ethanol and water act as an acid. The reaction between sodium and ethanol is less vigorous, so ethanol is a weaker acid than water.
• The Formation of an Esters
• Esterification reaction: Alcohol + Carboxylic acid → Ester + Water
• During this reaction, the bond between O–H in alcohol is broken
• Condition: Boiling the mixture under reflux
• Catalyst: Strong acid such as conc. sulfuric acid
• When ethanol is refluxed with ethanoic acid in the presence of a few drops of concentrated sulfuric acid (catalyst), an ester, ethyl ethanoate (fruity smell) is formed
• The Oxidation of Alcohols
• Oxidation agents: Hot acidified potassium dichromate (VI) solution or hot acidified potassium manganate (VII) solution
• The oxidation of the different alcohols links alcohols with aldehydes, ketones, and carboxylic acids: -Primary alcohol --> Aldehyde --> Carboxylic Acid
  • Ethanol reacts with acidified potassium dichromate(VI) under gentle heating to produce ethanal.
  • On further heating under reflux with an excess of acidified potassium dichromate(VI), the ethanal are oxidized to ethanoic acid.
  • Secondary alcohol --> Ketones(Resist further oxidation)
    • Propan-2-ol, on gentle heating with potassium dichromate (VI) acidified with sulfuric acid produces propanone.
  • Tertiary alcohol --> Resist further oxidation
    • 3° alcohols are resistant to oxidation because they do not have a hydrogen attached to the carbon bearing the hydroxyl group, -OH.

Fission of the carbon-oxygen bond (C-OH)

• The Dehydration of Alcohols

• An elimination reaction - organic molecule loses a hydrogen atom and a hydroxyl group to form a molecule of water (dehydration). Can be carried out by either:

• Heating a mixture of an alcohol and concentrated sulfuric acid (catalyst) or concentrated phosphoric (v) acid at about 170 - 180°C.

• Passing the alcohol vapour over an aluminum oxide catalyst heated 350°C.

• Dehydration of alcohols using an acid catalyst - When Ethanol is heated to make Ethene.

• Dehydration of alcohols using aluminum oxide catalyst Aluminum oxide granules, ceramic wool is soaked in ethanol and heat and water.

• When passing ethanol to make ethene water

• Dehydration of 2° or 3° alcohols, will lead to the information of MORE THAN ONE possible alkenes and both cis and trans-isomers may be formed

• Reaction with Hydrogen Halides*

  • Alcohol reacts with hydrogen halides HCL,HBr,HI to form ALKYL HALIDES under reflux -The older of reactivity of alcohols is

      - tertiary > secondary > primary
     Reactivity with hydrogen halides decreases
    

• Reaction with Phosphorus Halides

  • Allcohols React with phosphorus(V) CHOLIRDE PCL5 TO FORM A CHLOROALKANE and hydrogen chloride gas (steamy fumes) CH3CH2-OH (i) + PCL5(9) - CH3CH2_CL(I) + POCI3(9) + HCL (g)

Lucas's test

• This is a test to distinguish between 1°, 2°, and 3° alcohols The reagent consists of mixture of concentrated hydrochloric acid and zinc chloride. The reactivity depends on the rate,
• The reactivity of the 3 classes of alcohols towards Lucas's reagent increases in the order: -primary < secondary < tertiary
• Lucas’s test
  • In Primary- Dissolve in Lucas reagent but does not react. -In secondary- Chloroalkanes are formed after slight warming.
  • In Tertiary- Chloroalkanes are formed instantly, producing cloudiness immediately.