Alkyl Halides and Alcohols
🟢 Lite — Quick Review (1h–1d)
Rapid summary for last-minute revision before your exam.
Alkyl Halides and Alcohols — Key Facts Alkyl halides: R-X where X = F, Cl, Br, I (functional group: carbon-halogen bond) Classification: primary (1°) — X attached to C with 1 R group; secondary (2°) — 2 R groups; tertiary (3°) — 3 R groups Alcohols: R-OH; classification same as alkyl halides IUPAC: alkyl halides = haloalkane (e.g., 2-bromopropane = CH₃CHBrCH₃); alcohols = alkanol (e.g., ethanol = C₂H₅OH) ⚡ Exam tip: SN2 rate order: methyl > 1° > 2° > 3° (steric hindrance); SN1 rate order: 3° > 2° > 1° > methyl
🟡 Standard — Regular Study (2d–2mo)
Standard content for students with a few days to months.
Alkyl Halides and Alcohols — JAMB Chemistry Study Guide
Alkyl halide reactions:
- Nucleophilic substitution: $R-X + OH^- \rightarrow R-OH + X^-$ (hydrolysis)
- Elimination (E2): with strong base, forms alkene; reactivity: 3° > 2° > 1°
- Reaction with NaOH/aq KOH: gives alcohol; with alcoholic KOH: gives alkene (dehydrohalogenation)
- Grignard formation: $R-X + Mg \xrightarrow{dry\ ether} R-Mg-X$ (organomagnesium compound)
Alcohol reactions:
- Combustion: $C_2H_5OH + 3O_2 \rightarrow 2CO_2 + 3H_2O$ (complete)
- Dehydration: $conc\ H_2SO_4, 170°C \rightarrow alkene$; $conc\ H_2SO_4, 140°C \rightarrow ether$
- Oxidation: $RCH_2OH \xrightarrow{[O]} RCHO \xrightarrow{[O]} RCOOH$ (primary); $R_2CHOH \xrightarrow{[O]} ketone$ (secondary); tertiary alcohol resists oxidation
- Esterification: $RCOOH + R’OH \rightleftharpoons RCOOR’ + H_2O$
Comparisons:
| Property | CH₃Cl | CH₃Br | CH₃I |
|---|---|---|---|
| Boiling point (°C) | -24 | 4 | 43 |
| Reason | Smallest MP, weakest London | Larger, stronger LDF | Largest MP, strongest LDF |
| Polarity (C-X) | Most polar | Less | Least |
Common student mistakes: Confusing SN1 and SN2 mechanisms and their conditions; forgetting that tertiary alkyl halides undergo SN1 (carbocation intermediate) while primary undergoes SN2; not knowing why alcoholic KOH promotes elimination while aqueous KOH promotes substitution.
🔴 Extended — Deep Study (3mo+)
Comprehensive coverage for students on a longer study timeline.
Alkyl Halides and Alcohols — Comprehensive Chemistry Notes
SN1 mechanism (unimolecular nucleophilic substitution): Two-step process for tertiary alkyl halides: Step 1 (slow, rate-determining): $R_3C-X \rightarrow R_3C^+ + X^-$ (carbocation formation) Step 2 (fast): $R_3C^+ + OH^- \rightarrow R_3C-OH$ Characteristics: rate depends only on alkyl halide (first order); intermediate is planar carbocation; racemisation occurs at stereocentre.
SN2 mechanism (bimolecular nucleophilic substitution): One-step concerted process for methyl and primary alkyl halides: Backside attack of nucleophile; transition state has pentavalent carbon; Walden inversion occurs (stereochemistry inverts). Characteristics: rate depends on both alkyl halide and nucleophile (second order); no intermediate; stereochemistry inverts.
E1 mechanism (unimolecular elimination): Similar to SN1 but base removes H⁺ from adjacent carbon; carbocation intermediate; common for tertiary alkyl halides with weak base/heating.
E2 mechanism (bimolecular elimination): One-step concerted: base abstracts H⁺ while leaving group departs simultaneously; anti-periplanar geometry preferred; common for secondary/tertiary with strong base.
Oxidation of alcohols — detail: Primary alcohol → aldehyde → carboxylic acid: $CH_3CH_2OH \xrightarrow{K_2Cr_2O_7/H_2SO_4} CH_3CHO \xrightarrow[K_2Cr_2O_7/H_2SO_4]{} CH_3COOH$ Note: to stop at aldehyde, use PCC (pyridinium chlorochromate) or pyridine/CrO₃ instead of acidic dichromate. Secondary alcohol → ketone: $(CH_3)_2CHOH \xrightarrow{K_2Cr_2O_7/H_2SO_4} (CH_3)_2C=O$ Tertiary alcohol → no reaction under mild oxidation (must break C-C bonds).
Polyhalogenated compounds: Chloroform ($CHCl_3$): anaesthetic; decomposes in light to produce phosgene ($COCl_2$); stored in dark bottles with ethanol. Carbon tetrachloride ($CCl_4$): historically used as cleaning solvent; toxic to liver and kidneys; freons (CFCs) used as refrigerants (destroy ozone layer).
JAMB exam patterns:
- 2022 JAMB: 2-bromopropane reacts with alcoholic KOH; name the major product
- 2021 JAMB: Which of CH₃OH, C₂H₅OH, and C₃H₇OH will oxidise to a carboxylic acid?
- 2020 JAMB: Explain why tertiary butyl bromide undergoes hydrolysis mainly by SN1 mechanism
- 2019 JAMB: Write equation for reaction between ethyl bromide and aqueous sodium hydroxide
📊 JAMB Exam Essentials
| Detail | Value |
|---|---|
| Questions | 180 MCQs (UTME) |
| Subjects | 4 subjects (language + 3 for course) |
| Time | 2 hours |
| Marking | +1 per correct answer |
| Score | 400 max (used for university admission) |
| Registration | January – February each year |
🎯 High-Yield Topics for JAMB
- Use of English (Grammar + Comprehension) — 60 marks
- Biology for Science students — 40 marks
- Chemistry (Organic + Physical) — 40 marks
- Physics (Mechanics + Optics) — 35 marks
- Mathematics (Algebra + Geometry) — 40 marks
📝 Previous Year Question Patterns
- Q: “The process of photosynthesis requires…” [2024 Biology]
- Q: “The electronic configuration of Fe is…” [2024 Chemistry]
- Q: “Find the value of x if 2x + 5 = 15…” [2024 Mathematics]
💡 Pro Tips
- Use of English carries the most weight — master grammar rules and comprehension strategies
- JAMB syllabus is your Bible — questions come directly from it. Download and use it.
- Past questions are highly predictive — repeat patterns appear every year
- For Science students, Biology and Chemistry are high-scoring if you study NCERT-level content
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📐 Diagram Reference
Clear scientific diagram of Alkyl Halides and Alcohols with atom labels, molecular structure, reaction arrows, white background, color-coded bonds and groups, exam textbook style
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