Numford
Organic conversion

Ethanol to but-1-yne

4 steps: elimination, then addition, then elimination, then substitution

Class 12, Chapter 7 Alcohols, Phenols and Ethers; Class 11, Chapter 9 Hydrocarbons

  1. OH
    EthanolCH₃CH₂OHAlcohol
  2. Step 1

    conc. H₂SO₄

    443 K

    i

    Dehydration at 443 K gives the alkene; at 413 K with excess ethanol the product is the ether instead.

    EtheneCH₂=CH₂Alkene
  3. Step 2

    Br₂ in CCl₄

    i

    Red-brown bromine is decolourised: the test for unsaturation.

    BrBr
    1,2-DibromoethaneBrCH₂CH₂BrHaloalkane
  4. Step 3

    (i) alc. KOH, heat (ii) NaNH₂

    i

    Vicinal dihalide loses two HBr, first with alcoholic KOH and then with sodamide.

    EthyneHC≡CHAlkyne
  5. Step 4

    (i) NaNH₂ (ii) CH₃CH₂Br

    i

    Sodium acetylide (ethyne + NaNH₂) with a primary alkyl halide gives the higher alkyne; NCERT shows the acetylide and says alkynides are used to make higher alkynes. Standard board answer.

    But-1-yneCH₃CH₂C≡CHAlkyne

Exam answer

  1. Step 1: Ethanol to ethene: conc. H₂SO₄, 443 K.
  2. Step 2: Ethene to 1,2-dibromoethane: Br₂ in CCl₄.
  3. Step 3: 1,2-Dibromoethane to ethyne: (i) alc. KOH, heat (ii) NaNH₂.
  4. Step 4: Ethyne to but-1-yne: (i) NaNH₂ (ii) CH₃CH₂Br.
Why these reagents

Reagent and reason, step by step

  • Step 1: Ethanol to ethene: conc. H₂SO₄, 443 K Dehydration at 443 K gives the alkene; at 413 K with excess ethanol the product is the ether instead.
  • Step 2: Ethene to 1,2-dibromoethane: Br₂ in CCl₄ Red-brown bromine is decolourised: the test for unsaturation.
  • Step 3: 1,2-Dibromoethane to ethyne: (i) alc. KOH, heat (ii) NaNH₂ Vicinal dihalide loses two HBr, first with alcoholic KOH and then with sodamide.
  • Step 4: Ethyne to but-1-yne: (i) NaNH₂ (ii) CH₃CH₂Br Sodium acetylide (ethyne + NaNH₂) with a primary alkyl halide gives the higher alkyne; NCERT shows the acetylide and says alkynides are used to make higher alkynes. Standard board answer.
Good to know

Points to note

  • Step 1Dehydration at 443 K gives the alkene; at 413 K with excess ethanol the product is the ether instead.
Questions

Ethanol to but-1-yne: quick answers

How many steps does it take to convert ethanol to but-1-yne?

4 steps: Ethanol, then ethene, then 1,2-dibromoethane, then ethyne, then but-1-yne.

Which named reaction is used to convert ethanol to but-1-yne?

None of the steps has a name. They are elimination and addition and elimination and substitution, so write the reagent and the condition for each step.

What is the reagent for step 1 of ethanol to but-1-yne?

Ethanol to ethene: conc. H₂SO₄, 443 K. Dehydration at 443 K gives the alkene; at 413 K with excess ethanol the product is the ether instead.

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