Presentation on theme: "Reactions of alkynes Lec.7. Reactions of alkynes Alkynes undergo many of the same electrophilic additions as alkenes. 1-Addition of Hydrogen Halides and."— Presentation transcript:
Reactions of alkynes Lec.7
Reactions of alkynes Alkynes undergo many of the same electrophilic additions as alkenes. 1-Addition of Hydrogen Halides and Addition of Halogens In the first step of the reaction of an alkyne with a hydrogen halide, the electrophilic[ H + ] adds to the alkyne. If the alkyne is a terminal alkyne, the [ H + ] will add to the carbon that rich with hydrogen(Obey Markovinkov's rule.)
Alkynes add either 1 or 2 moles of HBr and HCl, depending on the ratios in which the alkyne and halogen acid are mixed. The mechanism of halogen addition:
Addition of halogen : Note: Vinylic carbocation :A carbocation in which the positive charge is on one of the carbons of a carbon-carbon double bond.
2-Addition of hydrogen: The product is a cis alkene, because both hydrogens add to the same face of the triple bond. Note: The Lindlar`s catalyst is a finely powdered palladium metal deposited on solid calcium carbonate that has been specially modified with lead salts.
The product of the reaction is an enol. An enol has a carbon–carbon double bond and an OH group bonded to one of carbon that has the triple bond. The enol[-ene for C=C,and ol for (OH)] immediately rearranges to a ketone. A ketone is a compound that has two alkyl groups bonded to a carbonyl group(A carbon doubly bonded to an oxygen) 3-Addition of Water(The product is ketone) Note: Hydration means addition of water, so dehydration means removal of water.
4- Acidity of Alkynes A hydrogen atom on a triply bonded carbon is weakly acidic and can be removed by a very strong base. Sodium amide, for example, converts acetylenes to acetylides.
Commercial Use of Ethyne(acetelyne ) Most of the ethyne produced commercially is used as a starting material for polymers. Polymers are large molecules that are made by linking together many small molecules. The small molecules used to make polymers are called monomers. Ex: vinyl chloride Straight-chain poly(vinyl chloride) is hard and rather brittle; branched poly(vinyl chloride) is the soft elastic vinyl commonly used both as a substitute for leather and in the manufacture of such things as garbage bags and shower curtains
Revision Give IUPAC names for the following: H 2 C=CHCH 2 CH 2 CH=CHCH 3 CH 3 CH═CHCH 2 C≡CCH 3 (CH 3 ) 3 CCH 2 CH 2 CH 2 CH 2 CH 3 CH 3 C ≡ CCH(Cl)CH 2 CH 3
2,2-dimethyl-4-propyloctane 1-ethyl-3-methylcyclopentane 3-methyl-3-heptene 1,1-DimethylCycloButane 4,5-dimethyl-2-heptyne Classify the following reactions as Addition, Elimination, substitution or rearrangement: (A)-CH3Br+KOH CH 3 OH+KBr (B)-CH 3 CH 2 OH CH 2 =CH 2 + H 2 O (C)- CH 2 =CH 2 +H 2 CH 3 CH 3 Draw the structure formula for the following: