Students can go through TS Inter 2nd Year Chemistry Notes 11th Lesson Haloalkanes and Haloarenes will help students in revising the entire concepts quickly.
TS Inter 2nd Year Chemistry Notes 11th Lesson Haloalkanes and Haloarenes
→ Alkyl/aryl halides may be classified as mono, di or polyhalogen (tri-tetra, etc) compounds depending upon whether they contain one, two or more halogen atoms in their structures.
→ Halogen compounds containing sp3 C – X bond (X = F, Cl, Br, I) include Alkyl halides (haloalkanes), Allylic halides and Benzylic halides.
→ Alkyl halides are further classified as pri-mary (1°). secondary (2°) or tertiary (3°) according to the nature of the carbon to which the halogen is attached.
→ Halogen compounds containing sp2 C – X bond include Vinylic halides and Aryl halides.
→ Dihaloalkanes are called geminal (gem) halides if halogen atoms are present in the same carbon atom and vicinal (vie) halides if halogen atoms are present on adjacent carbon atoms.
→ Since halogen atoms are more electronegative than carbon, the carbon-halogen bond of alkyl halide is polarised.
→ C – X bond length increases from C – F to C – I.
→ Alkyl halides are prepared from alcohols on reaction with concentrated halogen acids (HX), PCl3, PCl5 Or SOCl2.
→ Free radical chlorination or bromination of alkanes gives a complex mixture of isomeric mono and polyhaloalkanes.
→ Aryl chlorides or bromides can be prepared by reaction of the hydrocarbon with chlorine or bromine at room temperature in the presence of halogen carrier like Fe, AlCl3 or SbCl5.
→ When an aromatic primary amine reacts with nitrous acid (NaNO2 + HX) at 0 – 5°C, a diazonium salt is formed. The diazonium salt solution reacts with cuprous chloride or cuprous bromide-, the diazonium group is replaced by – Cl or – Br. This reaction is known as Sandmeyer s reaction.
→ When diazonium salt solution is shaken with KI solution the diazonium group is replaced by iodine.
→ Addition of bromine in CCl4 to an alkene results in discharge of reddish brown colour of bromine. This is an important test for the detection of unsaturation in a molecule.
→ Alkyl fluoride is prepared by heating an alkyl chloride or bromide in the presence of a metallic fluoride such as AgF2, Hg2F2 etc (swarts reaction).
CH3Br + AgF → CH3F + AgBr
→ Haloalkanes are only very slightly soluble in water.
→ The chemical reactions of haloalkanes may be divided into three categories.
- Nucleophilic substitution
- Elimination reactions
- Reaction with metals.
→ The order of reactivity of alkyl halides towards SN1 and SN2 reactions is as follows :
- For SN2 reaction : CH3X > primary halide > secondary halide > tertiary halide.
- For SN1 reaction: tertiary halide > secondary halide > primary halide > CH3X.
→ A carbon atom joined to four different atoms or groups is called an asymmetric carbon or stereocentre or chirality centre.
→ The objects which are non-superimposable on their mirror images are said to be chiral and this property is known as chirality. Objects which are superimposable on their mirror images are called achiral.
→ Optical activity is the ability of a chiral sub-stance to rotate the plane of plane-polarised light.
→ The three dimensional arrangement of atoms or groups at a chirality centre is called the absolute configuration.
→ The stereoisomers related to each other as non-superimposable mirror images are called enantiomers.
→ SN2 reactions of optically active alkyl halides are accompanied by inversion of configuration.
→ SN1 reactions of optically active alkyl halides are accompanied by racemisation.
→ Zaitsev rule (Saytzef rule) : “In dehydro- halogenation reactions, the preferred product is that alkene which has greater number of alkyl groups attached to the doubly bonded carbon atoms”.
→ Alkyl magnesium halides (RMgX) are known as Grignard reagents.
→ A number of polyhalogen compounds e.g., dichloromethane, chloroform, iodoform, CCl4, freon and DDT have many industrial applications.
→ Chloroform is slowly oxidised by air in the presence of light to a very poisonous gas, carbonyl chloride, also known as phosgene, COCl2. Hence chloroform is stored in closed amber coloured bottles completely filled so that air is kept out.
→ Dichlorodifluoro methane (CCl2F2) is known as Freon 12.
→ Paul Muller discovered the effectiveness of DDT as an insecticide.