Preparation by reaction at a functional group
Classifying organic reactions by type highlights similarities between reactions that may at first appear different, and makes learning reaction chemistry easier. The reaction of an alkane with halogen to form a haloalkane is an example of a substitution reaction (A substitution reaction of an organic molecule is one in which a part of the organic molecule is changed for a part of the reagent. Two products result (RH + Cl–Cl → RCl + HCl). ) at saturated carbon.
substitution
substitution
CH3–CH2–H + Cl–Cl 
CH3CH2–Cl + H–Cl
The reaction of an alkane with an elemental halogen to form a haloalkane (alkyl (An alkane minus one hydrogen atom. Examples: CH3 (methyl) and CH3CH2 (ethyl). Alkyl groups are often represented by R when showing general formulae such as RCO2H or RX (X = Cl,Br).) halide) is unusual as reaction can, in principle occur at any carbon.The most useful preparations of haloalkanes are those that result in introduction of halogen at specific carbon atoms with no change at other carbons. Examples of these are the addition (In addition reactions of organic substrates the two reactants combine to give a single product with no atoms left over.) and substitution reactions (A substitution reaction of an organic molecule is one in which a part of the organic molecule is changed for a part of the reagent. Two products result (RH + Cl–Cl → RCl + HCl). ) given below.
addition
Alkenes/alkynes react with halogen (X2) or HX in the absence of light at multiply bonded carbons.
CH2=CH2 + Br–Br → Br–CH2–CH2–Br
CH2=CH2 + H–Cl → H–CH2–CH2–Cl
substitution
Alcohols react with a variety of halogen-containing reagents to substitute the OH for halogen. Common ones are SOCl2, PCl3 (PBr3), PCl5, conc HCl (HBr).
CH3CH2–Cl + H–ClThe reaction of an alkane with an elemental halogen to form a haloalkane (alkyl (An alkane minus one hydrogen atom. Examples: CH3 (methyl) and CH3CH2 (ethyl). Alkyl groups are often represented by R when showing general formulae such as RCO2H or RX (X = Cl,Br).) halide) is unusual as reaction can, in principle occur at any carbon.The most useful preparations of haloalkanes are those that result in introduction of halogen at specific carbon atoms with no change at other carbons. Examples of these are the addition (In addition reactions of organic substrates the two reactants combine to give a single product with no atoms left over.) and substitution reactions (A substitution reaction of an organic molecule is one in which a part of the organic molecule is changed for a part of the reagent. Two products result (RH + Cl–Cl → RCl + HCl). ) given below.
addition
Alkenes/alkynes react with halogen (X2) or HX in the absence of light at multiply bonded carbons.
CH2=CH2 + Br–Br → Br–CH2–CH2–Br
CH2=CH2 + H–Cl → H–CH2–CH2–Cl
substitution
Alcohols react with a variety of halogen-containing reagents to substitute the OH for halogen. Common ones are SOCl2, PCl3 (PBr3), PCl5, conc HCl (HBr).
CH3CH2–OH → CH3CH2–Cl
CH3CH2–OH → CH3CH2–Br
CH3CH2–OH → CH3CH2–Br