ChemistryChapter 16: Hydrocarbons
Alkynes (school)
Alkynes: C triple C, CnH2n-2 (one triple).
Learning objectives
- Explain alkynes (school) in simple chemical language.
- Connect 1. structure with 2. reactions using equations, observations, or trends where needed.
- Answer a short Federal Board question with the correct terms and a clear reason.
Definition
Alkynes: C triple C, CnH2n-2 (one triple). Ethyne (acetylene) named. Linear at the triple bond (sp named). Add two lots of Br2 if excess (more unsaturation). Terminal alkynes weakly acidic named (not a synthesis of explosives). Combustion very sooty vs alkanes sometimes (more C). Meaning: same addition idea, extra pi bonds. C triple bond; ethyne linear; acidic H on terminal alkyne (HCCH) — weak acid named. Show curly arrows on addition mechanisms; Markovnikov reasoning must precede the product structure.
Important terms
- ALKYNES (SCHOOL)
- Alkynes: C triple C, CnH2n-2 (one triple).
- 1. STRUCTURE
- 180 degrees at C#C.
- 2. REACTIONS
- H2 excess -> alkane (named stages alkene then alkane if limited H2).
Relationship
- triple = more unsaturation
Overview
Shape, then addition:
Structure
180 degrees at C#C. Shorter stronger bond than C=C, but still reactive pi. Naming: -yne, number the triple. Two additions possible — first gives alkene, second alkane if excess reagent.
Examples Ethyne · But-2-yne named · Terminal vs internal
Reactions
H2 excess -> alkane (named stages alkene then alkane if limited H2). Br2 decolourise, more Br2 needed. Water addition named if the book has carbonyl from ethyne (school name only). Test: ethyne burns smoky flame; ammoniacal CuCl precipitate with terminal alkyne.
Examples More Br2 · Hydrogenate named · No welding-gas misuse
WHAT — ALKYNES (SCHOOL)
Alkynes: C triple C, CnH2n-2 (one triple). Ethyne (acetylene) named. Linear at the triple bond (sp named). Add two lots of Br2 if excess (more unsaturation). Terminal alkynes weakly acidic named (not a synthesis of explosives). Combustion very sooty vs alkanes sometimes (more C). Meaning: same addition idea, extra pi bonds. C triple bond; ethyne linear; acidic H on terminal alkyne (HCCH) — weak acid named. Show curly arrows on addition mechanisms; Markovnikov reasoning must precede the product structure.
WHY — Why this topic matters
ALKYNES (SCHOOL) is an independent FBISE learning unit because students must use its definitions, conditions, equations, or trends separately in explanations and calculations.
HOW — How it works
STRUCTURE: 180 degrees at C#C. Shorter stronger bond than C=C, but still reactive pi. Naming: -yne, number the triple. Two additions possible — first gives alkene, second alkane if excess reagent. Examples: Ethyne; But-2-yne named; Terminal vs internal. REACTIONS: H2 excess -> alkane (named stages alkene then alkane if limited H2). Br2 decolourise, more Br2 needed. Water addition named if the book has carbonyl from ethyne (school name only). Test: ethyne burns smoky flame; ammoniacal CuCl precipitate with terminal alkyne. Examples: More Br2; Hydrogenate named; No welding-gas misuse. Exam alkyne vs alkene: alkyne has a triple bond, more unsaturation can add more Br2 than an alkene both decolourise Br2; alkanes do not
In simple words
School alkynes. No welding or explosive acetylene how-to.
Key points
- triple = more unsaturation
- adds more Br2
- sp linear
- two pi bonds
Exam preparation
Exam Focus
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.Alkynes (school)
Sticky Notes · Quick Revision
Core Idea
- Alkynes: C triple C, CnH2n-2 (one triple). Ethyne (acetylene) named. Linear at the triple bond (sp named). Add two lots of Br2 if excess (more unsaturation). Terminal alkynes weakly acidic named (not a synthesis of explosives). Combustion very sooty vs alkanes sometimes (more C). Meaning: same addition idea, extra pi bonds. C triple bond; ethyne linear; acidic H on terminal alkyne (HCCH) — weak acid named. Show curly arrows on addition mechanisms; Markovnikov reasoning must precede the product structure.
1. STRUCTURE
- 180 degrees at C#C. Shorter stronger bond than C=C, but still reactive pi. Naming: -yne, number the triple. Two additions possible — first gives alkene, second alkane if excess reagent.
- Ethyne
- But-2-yne named
- Terminal vs internal
2. REACTIONS
- H2 excess -> alkane (named stages alkene then alkane if limited H2). Br2 decolourise, more Br2 needed. Water addition named if the book has carbonyl from ethyne (school name only). Test: ethyne burns smoky flame; ammoniacal CuCl precipitate with terminal alkyne.
- More Br2
- Hydrogenate named
- No welding-gas misuse
Exam Example
- Exam alkyne vs alkene:
- alkyne has a triple bond, more unsaturation
- can add more Br2 than an alkene
- both decolourise Br2; alkanes do not
Common Misconceptions
- Do not confuse 1. STRUCTURE with 2. REACTIONS; state the difference before giving an example.
- Writing only “triple = more unsaturation” is not enough. Explain what it means in a complete sentence.
FBISE Exam Check
- Explain alkynes (school), compare its two main parts, and include the correct chemical evidence.
- School alkynes. No welding or explosive acetylene how-to.
60-second revision
- Alkynes: C triple C, CnH2n-2 (one triple).
- 1. STRUCTURE: 180 degrees at C#C.
- 2. REACTIONS: H2 excess -> alkane (named stages alkene then alkane if limited H2).
- Exam reminder: Exam alkyne vs alkene: alkyne has a triple bond, more unsaturation can add more Br2 than an alkene both decolourise Br2; alkanes do not
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