ChemistryChapter 17: Halogenoalkanes
Classifying Halogenoalkanes
Replace H of an alkane by F, Cl, Br, I.
Learning objectives
- Explain classifying halogenoalkanes in simple chemical language.
- Connect 1. 1, 2, 3 with 2. polarity and b.p. using equations, observations, or trends where needed.
- Answer a short Federal Board question with the correct terms and a clear reason.
Definition
Replace H of an alkane by F, Cl, Br, I. Primary (1): C-X has two H (or is methyl). Secondary (2): one H on that C. Tertiary (3): no H, three carbons on that C. C-X is polar (C d+, X d-). Bond weaker down Cl Br I (I easiest to substitute in school). Meaning: class predicts SN1 vs SN2 later. These notes are school mechanisms, not illicit synthesis. Primary, secondary, tertiary by C bearing X — 0, 1, or 2 other carbons on that C. Classify the halogen-bearing carbon as 1, 2, or 3 before choosing SN1, SN2, or elimination.
Important terms
- CLASSIFYING HALOGENOALKANES
- Replace H of an alkane by F, Cl, Br, I.
- 1. 1, 2, 3
- Chloroethane 1.
- 2. POLARITY AND B.P.
- Polar molecules, dipole.
Overview
Count the carbons on C-X:
1, 2, 3
Chloroethane 1. 2-chloropropane 2. 2-chloro-2-methylpropane 3. Draw and count. Iodo compounds often fastest substitution (weaker C-I). R-X polarity: C delta+ X delta- — X is leaving group in substitution.
Examples Methyl is 1 · Isopropyl 2 · tert-butyl 3
Polarity and B.p.
Polar molecules, dipole. b.p. higher than the alkane of similar size (extra force). Insoluble in water like many organics. No solvent-abuse guide. 1-chlorobutane primary; 2-chlorobutane secondary; tert-butyl chloride tertiary.
Examples Heavier X, higher b.p. · Not hydrogen-bond donors like alcohols · Colourless liquids often
WHAT — CLASSIFYING HALOGENOALKANES
Replace H of an alkane by F, Cl, Br, I. Primary (1): C-X has two H (or is methyl). Secondary (2): one H on that C. Tertiary (3): no H, three carbons on that C. C-X is polar (C d+, X d-). Bond weaker down Cl Br I (I easiest to substitute in school). Meaning: class predicts SN1 vs SN2 later. These notes are school mechanisms, not illicit synthesis. Primary, secondary, tertiary by C bearing X — 0, 1, or 2 other carbons on that C. Classify the halogen-bearing carbon as 1, 2, or 3 before choosing SN1, SN2, or elimination.
WHY — Why this topic matters
CLASSIFYING HALOGENOALKANES 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
1, 2, 3: Chloroethane 1. 2-chloropropane 2. 2-chloro-2-methylpropane 3. Draw and count. Iodo compounds often fastest substitution (weaker C-I). R-X polarity: C delta+ X delta- — X is leaving group in substitution. Examples: Methyl is 1; Isopropyl 2; tert-butyl 3. POLARITY AND B.P.: Polar molecules, dipole. b.p. higher than the alkane of similar size (extra force). Insoluble in water like many organics. No solvent-abuse guide. 1-chlorobutane primary; 2-chlorobutane secondary; tert-butyl chloride tertiary. Examples: Heavier X, higher b.p.; Not hydrogen-bond donors like alcohols; Colourless liquids often. Exam class: look at the carbon holding X how many other carbons join it? 0 or 1 extra C with H2: primary idea; 3 carbons: tertiary
In simple words
Classification. No haloalkane manufacture.
Key points
- 12 / 3 at the C-X carbon
- C d+ is the attack site
- how many C on C-X
- C-X polar
Exam preparation
Exam Focus
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.Classifying Halogenoalkanes
Sticky Notes · Quick Revision
Core Idea
- Replace H of an alkane by F, Cl, Br, I. Primary (1): C-X has two H (or is methyl). Secondary (2): one H on that C. Tertiary (3): no H, three carbons on that C. C-X is polar (C d+, X d-). Bond weaker down Cl Br I (I easiest to substitute in school). Meaning: class predicts SN1 vs SN2 later. These notes are school mechanisms, not illicit synthesis. Primary, secondary, tertiary by C bearing X — 0, 1, or 2 other carbons on that C. Classify the halogen-bearing carbon as 1, 2, or 3 before choosing SN1, SN2, or elimination.
1. 1, 2, 3
- Chloroethane 1. 2-chloropropane 2. 2-chloro-2-methylpropane 3. Draw and count. Iodo compounds often fastest substitution (weaker C-I). R-X polarity: C delta+ X delta- — X is leaving group in substitution.
- Methyl is 1
- Isopropyl 2
- tert-butyl 3
2. POLARITY AND B.P.
- Polar molecules, dipole. b.p. higher than the alkane of similar size (extra force). Insoluble in water like many organics. No solvent-abuse guide. 1-chlorobutane primary; 2-chlorobutane secondary; tert-butyl chloride tertiary.
- Heavier X, higher b.p.
- Not hydrogen-bond donors like alcohols
- Colourless liquids often
Exam Example
- Exam class:
- look at the carbon holding X
- how many other carbons join it?
- 0 or 1 extra C with H2: primary idea; 3 carbons: tertiary
Common Misconceptions
- Do not confuse 1. 1, 2, 3 with 2. POLARITY AND B.P.; state the difference before giving an example.
- Writing only “12 / 3 at the C-X carbon” is not enough. Explain what it means in a complete sentence.
FBISE Exam Check
- Explain classifying halogenoalkanes, compare its two main parts, and include the correct chemical evidence.
- Classification. No haloalkane manufacture.
60-second revision
- Replace H of an alkane by F, Cl, Br, I.
- 1. 1, 2, 3: Chloroethane 1.
- 2. POLARITY AND B.P.: Polar molecules, dipole.
- Exam reminder: Exam class: look at the carbon holding X how many other carbons join it?
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