Alkenes and Alkynes
Preparation, electrophilic addition, Markovnikov's rule and peroxide effect; acidity of terminal alkynes. (Chemistry › Hydrocarbons, NEET UG syllabus.)
What is Alkenes and Alkynes?
In the electrophilic addition of an unsymmetrical reagent (like HX or H2O) to an unsymmetrical alkene or alkyne, the positive part of the reagent (H+) adds to the carbon atom of the double/triple bond that has a greater number of hydrogen atoms, and the negative part adds to the carbon atom with fewer hydrogen atoms.
Key formula / rule: Dehydration of Alcohol
Key points
- Describe various methods for the preparation of alkenes and alkynes.
- Explain the mechanism and regioselectivity of electrophilic addition reactions to alkenes and alkynes.
- State and apply Markovnikov's rule and the peroxide effect.
- Identify and explain the acidic nature of terminal alkynes and their characteristic reactions.
Common exam trap
Confusing Markovnikov's rule with anti-Markovnikov's rule and applying peroxide effect to HCl or HI.
Definitions
- Term
Markovnikov's Rule
- Meaning
In the electrophilic addition of an unsymmetrical reagent (like HX or H2O) to an unsymmetrical alkene or alkyne, the positive part of the reagent (H+) adds to the carbon atom of the double/triple bond that has a greater number of hydrogen atoms, and the negative part adds to the carbon atom with fewer hydrogen atoms.
- Term
Peroxide Effect (Anti-Markovnikov's Rule)
- Meaning
The reversal of Markovnikov's rule observed specifically during the addition of HBr to unsymmetrical alkenes in the presence of peroxides. In this free radical mechanism, the bromine atom adds to the carbon atom of the double bond that has a greater number of hydrogen atoms.
- Term
Electrophilic Addition
- Meaning
A type of addition reaction in organic chemistry where an electrophile (electron-deficient species) attacks an electron-rich π bond (in alkenes or alkynes), leading to the formation of a saturated product.
- Term
Terminal Alkyne
- Meaning
An alkyne in which the triple bond is at the end of the carbon chain, meaning one of the sp-hybridized carbons is bonded to a hydrogen atom (R-C≡C-H). These alkynes exhibit acidic properties.
- Term
Zaitsev's Rule
- Meaning
In an elimination reaction, the major product is the most substituted alkene, i.e., the alkene with the greatest number of alkyl groups attached to the double-bonded carbon atoms.
Learning objectives
Describe various methods for the preparation of alkenes and alkynes.
Explain the mechanism and regioselectivity of electrophilic addition reactions to alkenes and alkynes.
State and apply Markovnikov's rule and the peroxide effect.
Identify and explain the acidic nature of terminal alkynes and their characteristic reactions.
Distinguish between terminal and non-terminal alkynes using chemical tests.
Predict the major products of reactions involving alkenes and alkynes.
Formulae
- Name
Dehydration of Alcohol
- Note
Follows Zaitsev's rule for major product
- Expression
R-CH(OH)-CH2-R' --(conc. H2SO4/heat)-→ R-CH=CH-R' + H2O
- Name
Dehydrohalogenation of Alkyl Halide
- Note
Follows Zaitsev's rule for major product
- Expression
R-CH(X)-CH2-R' --(alc. KOH/heat)-→ R-CH=CH-R' + KX + H2O
- Name
Hydrogenation of Alkene
- Note
Forms alkane
- Expression
R-CH=CH-R' + H2 --(Ni/Pt/Pd)-→ R-CH2-CH2-R'
- Name
Partial Hydrogenation of Alkyne (cis)
- Note
Stereoselective synthesis of cis-alkene
- Expression
R-C≡C-R' + H2 --(Lindlar's catalyst)-→ cis-R-CH=CH-R'
- Name
Partial Hydrogenation of Alkyne (trans)
- Note
Stereoselective synthesis of trans-alkene
- Expression
R-C≡C-R' + Na/liq. NH3 -→ trans-R-CH=CH-R'
- Name
Addition of HX to Alkene (Markovnikov)
- Note
X adds to the carbon with fewer hydrogens
- Expression
R-CH=CH2 + HX -→ R-CH(X)-CH3
- Name
Addition of HBr to Alkene (Anti-Markovnikov)
- Note
Br adds to the carbon with more hydrogens, free radical mechanism
- Expression
R-CH=CH2 + HBr --(peroxides)-→ R-CH2-CH2-Br
- Name
Hydration of Alkyne
- Note
Forms enol intermediate, tautomerizes to ketone (except ethyne which forms acetaldehyde)
- Expression
R-C≡CH + H2O --(H2SO4/HgSO4)-→ [R-C(OH)=CH2] -→ R-CO-CH3
- Name
Acidity of Terminal Alkyne
- Note
Formation of sodium alkynide
- Expression
R-C≡CH + NaNH2 -→ R-C≡C-Na+ + NH3
Prerequisites
Basic organic nomenclature (alkanes, alkenes, alkynes, alcohols, alkyl halides).
Understanding of σ and π bonds, hybridization (sp, sp2, sp3).
Concepts of nucleophiles and electrophiles.
Basic reaction mechanisms (carbocation stability, free radicals).
Common mistakes
Confusing Markovnikov's rule with anti-Markovnikov's rule and applying peroxide effect to HCl or HI.
Forgetting that the peroxide effect is a free radical mechanism.
Not recognizing terminal alkynes and their acidic nature.
Incorrectly predicting products for hydration of alkynes (missing tautomerization step).
Mixing up catalysts for cis- and trans-alkene formation from alkynes.
Keywords
Alkenes
Alkynes
Electrophilic addition
Markovnikov's Rule
Peroxide Effect
Anti-Markovnikov
Terminal Alkyne
Acidity
Dehydration
Dehydrohalogenation
Hydrogenation
Lindlar's catalyst
Tollen's reagent
Zaitsev's Rule
Practice preview
Which of the following reagents is typically used for the dehydrohalogenation of an alkyl halide to form an alkene?…
easy
Which of the following types of hydrocarbons exhibits acidic character due to the presence of a terminal hydrogen atom?…
easy
What is the major product formed when HBr is added to propene (CH₃-CH=CH₂) in the absence of peroxides?…
medium