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Reactions of alkynes

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

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Reactions of alkynes

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  1. Reactions of alkynes Lec.7

  2. 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.)

  3. 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:

  4. 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.

  5. 2-Addition of hydrogen: The product is a cisalkene, 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.

  6. 3-Addition of Water(The product is ketone) • The product of the reaction is an enol. An enolhas 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,andol for (OH)]immediately rearranges to a ketone. • A ketoneis a compound that has two alkyl groups bonded to a carbonyl group(A carbon doubly bonded to an oxygen) Note: Hydration means addition of water, so dehydration means removal of water.

  7. 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.

  8. 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-chainpoly(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

  9. Revision • Give IUPAC names for the following: H2C=CHCH2CH2CH=CHCH3 CH3CH═CHCH2C≡CCH3 (CH3)3CCH2CH2 CH2 CH2CH3 CH3C ≡ CCH(Cl)CH2CH3

  10. Draw the structure formula for the following: 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 CH3OH+KBr (B)-CH3CH2OH CH2=CH2 + H2O (C)- CH2=CH2 +H2CH3CH3

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