Haloalkanes and Haloarenes

Flashcards on Haloalkanes and Haloarenes

Halogenation

The replacement of hydrogen atom(s) in an aliphatic or aromatic hydrocarbon by halogen atom(s).

Haloalkanes

Contain halogen atom(s) attached to the sp3 hybridised carbon atom of an alkyl group.

Haloarenes

Contain halogen atom(s) attached to sp2 hybridised carbon atom(s) of an aryl group.

Mono, di, or polyhalogen compounds

Compounds containing one, two, or more halogen atoms in their structures.

Alkyl halides or haloalkanes (R—X)

Halogen atom is bonded to an alkyl group (R).

Primary alkyl halide or 1° alkyl halide

Halogen is attached to a primary carbon atom in an alkyl halide.

Secondary alkyl halide (2°)

Halogen is attached to secondary carbon atom in an alkyl halide.

Tertiary alkyl halide (3°)

Halogen is attached to tertiary carbon atom in an alkyl halide.

Allylic halides

Halogen atom is bonded to an sp3-hybridised carbon atom adjacent to carbon-carbon double bond (C=C) i.e. to an allylic carbon.

Benzylic halides

Halogen atom is bonded to an sp3-hybridised carbon atom attached to an aromatic ring.

Vinylic halides

Halogen atom is bonded to a sp2-hybridised carbon atom of a carbon-carbon double bond (C = C).

Aryl halides

Halogen atom is directly bonded to the sp2-hybridised carbon atom of an aromatic ring.

Common names of alkyl halides

Naming the alkyl group followed by the name of halide.

IUPAC system of nomenclature for alkyl halides

Alkyl halides are named as halosubstituted hydrocarbons.

Nomenclature for benzene derivatives

Mono halogen substituted derivatives of benzene, common and IUPAC names are the same.

Alkylidene or alkylene dihalides

The dihaloalkanes having the same type of halogen atoms.

Geminal halides or gem-dihalides

Both the halogen atoms are present on the same carbon atom.

Vicinal halides or vic-dihalides

Halogen atoms are present on adjacent carbon atoms.

Halogen atoms

More electronegative than carbon, therefore, carbon-halogen bond of alkyl halide is polarised.

Alkyl halides

Best prepared from alcohols, which are easily accessible.

Preparation of haloalkanes from alkanes

Free radical chlorination or bromination of alkanes gives a complex mixture of isomeric mono- and polyhaloalkanes.

Preparation of haloalkanes from alcohols

The hydroxyl group of an alcohol is replaced by halogen on reaction with concentrated halogen acids, phosphorus halides or thionyl chloride.

Reaction with thionyl chloride

Alkyl halide is formed along with gases SO2 and HCl, escapable, giving pure alkyl halides.

Addition of hydrogen halides in alkenes

An alkene is converted to corresponding alkyl halide by reaction with hydrogen chloride, hydrogen bromide or hydrogen iodide.

Addition of halogens to alkenes

Addition of bromine in CCl4 to an alkene resulting in discharge of reddish brown colour of bromine.

Finkelstein reaction

Alkyl iodides are prepared by the reaction of alkyl chlorides/ bromides with NaI in dry acetone.

Swarts reaction

Synthesis of alkyl fluorides accomplished by heating an alkyl chloride/bromide in the presence of a metallic fluoride.

Preparation of haloarenes from hydrocarbons

Aryl chlorides and bromides prepared by electrophilic substitution of arenes with chlorine and bromine respectively.

Sandmeyer’s reaction

A primary aromatic amine, dissolved or suspended in cold aqueous mineral acid, is treated with sodium nitrite, a diazonium salt is formed.

Organic halogen compounds Polarity

Molecules of organic halogen compounds are generally polar.

Haloalkanes Solubility

Haloalkanes are very slightly soluble in water.

Nucleophilic substitution reaction

A nucleophile replaces already existing nucleophile in a molecule.

Ambident nucleophiles

Groups like cyanides and nitrites possess two nucleophilic centres.

Substitution nucleophilic bimolecular (SN2)

Incoming nucleophile interacts with alkyl halide causing the carbon-halide bond to break and a new bond is formed between carbon and attacking nucleophile.

Configuration

Spacial arrangement of functional groups around carbon.

Substitution nucleophilic unimolecular (SN1)

Reactions are generally carried out in polar protic solvents (like water, alcohol, acetic acid, etc.).

Optical activity

Plane of plane polarised light is rotated when it is passed through the solutions of certain compounds.

Polarimeter

Instrument to measured the angle by which the plane polarised light is rotated

Dextrorotatory

Rotates the plane of plane polarised light to the right, i.e., clockwise direction.

Laevo-rotatory

The light is rotated towards left (anticlockwise direction).

Enantiomers

Related to each other as non- superimposable mirror images.

Racemic mixture

A mixture containing two enantiomers in equal proportions will have zero optical rotation.

Racemisation

The process of conversion of enantiomer into a racemic mixture

Retention of configuration

Preservation of the spatial arrangement of bonds to an asymmetric centre during a chemical reaction or transformation.

Elimination reactions

When a haloalkane with -hydrogen atom is heated with alcoholic solution of potassium hydroxide, there is elimination of hydrogen atom from -carbon and a halogen atom from the -carbon atom.

Organo-metallic compounds

Organic chlorides, bromides and iodides react with certain metals to give compounds containing carbon-metal bonds.

Grignard Reagents

Alkyl magnesium halide, RMgX.

Wurtz reaction

Alkyl halides react with sodium in dry ether to give hydrocarbons containing double the number of carbon atoms present in the halide.

Reactions of Haloarenes

Haloarenes are extremely less reactive towards nucleophilic substitution reactions.

Wurtz-Fittig reaction

A mixture of an alkyl halide and aryl halide gives an alkylarene when treated with sodium in dry ether.

Fittig reaction

Aryl halides also give analogous compounds when treated with sodium in dry ether, in which two aryl groups are joined together.

Polyhalogen compounds

Carbon compounds containing more than one halogen atom.

Freons

The chlorofluorocarbon compounds of methane and ethane.