Alkynes Reactions and Nomenclature

Reactions with Alkynes

Elimination (Synthesis)
- Reagents: $XS\,NaNH2$ , then $\text{H}2\text{O}$
- Product: Alkyne from vicinal or geminal dihalide.
- Stereospecificity: Anti
- Mechanism: Yes, curved arrows.
 - First step (PT & LOLG): -NH2 is the base, β H is the acid
 - Second step (PT & LOLG): -NH2 is the base, β H is the acid
 - Third step (terminal alkyne, PT): -NH2 is the base, terminal H is the acid
 - Fourth step (terminal alkyne, PT): -C≡C is the base, H2O is the acid
- Regioselectivity: Neither
- Stereochemistry: Neither
- Additional Notes:
 - Two reagents needed
 - Concerted reaction
 - Requires an extra equivalent of sodium amide and a follow-up step to return a lost proton for terminal alkynes
Hydrohalogenation
- Reagents:
 - One equivalent: $HX$
 - Excess: $XS\,HX$
- Product: Alkyl halides (one or two equivalents).
- Stereospecificity: Anti
- Mechanism: No curved arrows.
 - First step (PT): H+ is the electrophile, pi bond is nucleophile
 - Unstable carbocation forms (no rearrangement)
 - Second step (NA): X- is the nucleophile, carbocation is electrophile
- Regioselectivity: Markovnikov
- Stereochemistry: Neither
- Additional Notes:
 - X can be Cl or Br
 - Pay attention to the quantity
 - 2° vinyl cations are relatively unstable, so they may not be formed during these addition reactions. The π-complex is an alternative possibility.
 - Electrophilic addition reactions that add a proton in the rate-determining step require a strong acid (typically, pKa < 0
Radical Hydrohalogenation
- Reagents:
  - One equivalent: $HX, ROOR$
  - Excess: $XS\,HX, ROOR$
- Product: Alkyl halides (one or two equivalents).
- Stereospecificity: Syn and Anti
- Mechanism: Chapter 10
- Regioselectivity: Anti-Markovnikov
- Stereochemistry: Racemic
- Additional Notes:
  - X can be Cl or Br
  - Pay attention to the quantity
  - Stereochemistry (E and Z)
Acid-Catalyzed Hydration
- Reagents: $\text{H}2\text{SO}4, \text{H}2\text{O}, HgSO4$
- Product: Ketone via Markovnikov addition.
- Stereospecificity: Neither
- Mechanism: Partial, includes tautomerization.
 - First step (PT): H+ is the acid, pi bond is nucleophile/base
 - Carbocation forms (no rearrangement – resonance stabilized)
 - Second step (PT): H2O is the nucleophile/base, H+ is the acid
- Regioselectivity: Markovnikov
- Stereochemistry: Neither
- Additional Notes:
 - Markovnikov alcohol → ketone
 - Tautomerization is acid catalyzed keto-enol tautomerization (acid). Tautomers are constitutional isomers that differ in the location of the double bond and hydrogen atom
Hydroboration-Oxidation
- Reagents: 1) $R2BH$ , 2) $\text{H}2\text{O}_2, NaOH$
- Product: Aldehyde via Anti-Markovnikov addition.
- Stereospecificity: Syn
- Mechanism: Partial, includes tautomerization.
 - First step (NA + PT): BH3 is the electrophile, pi bond is nucleophile
 - Concerted syn addition
 - First step (PT): alcohol is the acid, OH- is nucleophile/base
 - Enolate ion forms (no rearrangement – resonance stabilized)
 - Second step (PT): enolate ion is nucleophile/base, H2O is the acid
- Regioselectivity: Anti-Markovnikov
- Stereochemistry: Neither
- Additional Notes:
 - Regioselectivity (Anti-Markovnikov) alkene → alcohol → aldehyde
 - Tautomerization is base-catalyzed (OH-)
 - Bulky boron reagent needed for regioselectivity
 - Stereospecificity (syn)
Halogenation
- Reagents:
 - One equivalent: $X2, CCl4$
 - Excess: $XS\,X2, CCl4$
- Product: Halogenated alkyne (one or two equivalents).
- Stereospecificity: Anti
- Mechanism: Yes, curved arrows.
 - First step (NA + LOLG): Cl2 is the electrophile, pi bond is nucleophile
 - Cl- is lost as a LG, and a bridged intermediate Chloronium ion is formed
 - Second step (nucleophilic attack): Cl- (LG) is nucleophile, carbon with Chloronium ion is electrophile
 - REPEAT
- Regioselectivity: Neither
- Stereochemistry: Neither
- Additional Notes:
 - Can be Cl2 or Br2
 - Pay attention to the quantity
Ozonolysis
- Reagents: 1) $O3, CCl4$ , 2) $\text{H}_2\text{O}$
- Product: Carbonyl compounds
- Stereospecificity: Neither
- Mechanism: No curved arrows.
- Regioselectivity: Neither
- Stereochemistry: Neither
- Additional Notes:
 - Addition of carbonyls and breaking of sigma and pi C=C bonds
 - No carbocation generation (No rearrangement)
 - Stereospecificity (anti)
 - Stereochemistry (racemization)
Alkylation
- Reagents: 1) $NaNH2, NH3$ , 2) $RX$
- Product: Alkylated alkyne.
- Stereospecificity: Neither
- Mechanism: Yes, curved arrows.
 - First step (PT): –NH2 is the base, terminal H is the acid
 - Second step (NA & LOLG): –C≡C is the nucleophile, to the electrophilic carbon
- Regioselectivity: Neither
- Stereochemistry: Neither
- Additional Notes:
 - Terminal alkynes (only)
 - Methyl/1° substrate (SN2/nucleophile )
 - 2°/3° substrate (E2/base)
 - This is your first C-C bond-forming reaction!
 - It only works with primary or methyl leaving groups (SN2)
 - Secondary leaving groups undergo elimination
Hydrogenation
- Reagents:
  - Alkane: $\text{H}_2, Pt$
  - Cis-Alkene: $\text{H}_2$ , Lindlar's catalyst
  - Trans-Alkene: Na, $NH_3 (l)$
- Product: Alkane, cis-alkene, or trans-alkene.
- Stereospecificity: Syn
- Mechanism: No curved arrows.
- Regioselectivity: Neither
- Stereochemistry: Diastereoselective

Alkyne Nomenclature

Alkynes are named similarly to alkenes but end with –yne instead of –ene.
Number the chain to include both carbon atoms of C≡C.
Identify and name substituents.
- C≡C (or C=C) has the lowest locants.
- If equal, number alphabetically.
Assign a locant (and prefix if necessary) to each substituent, giving the C≡C triple bond the lowest number possible.
The locant for the alkyne suffix may precede the parent name or be placed immediately before the suffix.
List the numbered substituents before the parent name in alphabetical order. Ignore prefixes (except iso) when ordering alphabetically.
The C≡C triple bond locant is placed either just before the parent name or just before the –yne suffix.
Common names derived from acetylene are often used as well.
- Alkynes are also classified as terminal or internal.
Number substituted cycloalkenes to:
- Give the atoms of C≡C the 1 and 2 positions.
- Give the substituent groups the lower numbers at the first point of difference.
- Cycloalkanes with fewer than 8 carbons will not have an alkyne (too much strain).

Acidity and Basicity

Relative Acidity: H-OH > H-OR > H-C≡CR > H-NH2 > H-CH=CH2 > H-CH2CH3
$pK_a$ values: 15.7, 16-17, 25, 38, 44, 50
Relative Basicity: ^-OH < ^-OR < ^-C≡CR < ^-NH2 < ^-CH=CH2 < ^-CH2CH3