Organic Chemistry Fundamentals — Functional Groups

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Organic Chemistry Fundamentals — Functional Groups — Key Facts for Makerere University (Uganda) Core concept: Functional groups are specific atoms or groups of atoms within molecules that determine the chemical behavior and physical properties of organic compounds High-yield points: Recognize and name the major functional groups; understand their characteristic reactions; know the general formulas ⚡ Exam tip: Being able to identify functional groups by inspection is essential — questions frequently ask you to identify or name compounds based on their structural formulas

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Organic Chemistry Fundamentals — Functional Groups — Makerere University (Uganda) Study Guide

1. Introduction to Organic Chemistry

Organic chemistry is the study of carbon compounds. Carbon forms four covalent bonds, allowing it to chain, ring, and create millions of compounds.

Why carbon is special:

• Tetravalent (4 bonds per atom)
• Can bond to itself (C–C) forming long chains and rings
• Forms strong bonds with H, O, N, S, P, and halogens
• Multiple oxidation states allow diverse reactions
• catenation (ability to form long chains) and isomerism lead to enormous variety

Hydrocarbons

Compounds containing only carbon and hydrogen.

• Saturated hydrocarbons (Alkanes): C–C single bonds only, formula CₙH₂ₙ₊₂
• Unsaturated hydrocarbons: Contain C=C double bonds (Alkenes, CₙH₂ₙ) or C≡C triple bonds (Alkynes, CₙH₂ₙ₋₂)
• Aromatic hydrocarbons: Contain benzene ring (C₆H₆)

2. Functional Groups — Overview Table

Functional Group	Structure	Suffix	Prefix	Example	Boiling Point
Alkane	C–C	-ane	alkyl-	CH₄ (methane)	Very low
Alkene	C=C	-ene	alkenyl-	C₂H₄ (ethene)	Low
Alkyne	C≡C	-yne	alkynyl-	C₂H₂ (ethyne)	Low
Alcohol	–OH	-ol	hydroxy-	C₂H₅OH (ethanol)	Moderate-High
Ether	C–O–C	ether	alkoxy-	CH₃OCH₃ (dimethyl ether)	Low
Aldehyde	–CHO	-al	oxo-	CH₃CHO (acetaldehyde)	Moderate
Ketone	–CO–	-one	oxo-	CH₃COCH₃ (acetone)	Moderate
Carboxylic Acid	–COOH	-oic acid	carboxy-	CH₃COOH (acetic acid)	High
Ester	–COO–	-oate	alkanoyloxy-	CH₃COOCH₃ (methyl ethanoate)	Moderate
Amine	–NH₂	-amine	amino-	CH₃NH₂ (methylamine)	Low-Moderate
Amide	–CONH₂	-amide	amido-	CH₃CONH₂ (acetamide)	High
Nitro	–NO₂	—	nitro-	CH₃NO₂ (nitromethane)	Moderate
Halide	–CX (X=Cl,Br,F,I)	—	halo-	CH₃Cl (chloromethane)	Low

3. Detailed Functional Group Chemistry

3.1 Alkenes (C=C Double Bond)

General formula: CₙH₂ₙ Hybridization: Each C in the double bond is sp² hybridized Bond angle: ~120° π bond: Formed by sideways overlap of unhybridized p orbitals

Nomenclature: Root + -ene + position number

• ethene (CH₂=CH₂), propene (CH₃–CH=CH₂), but-1-ene, but-2-ene

Geometric Isomerism: Alkenes show cis-trans (E/Z) isomerism when each double-bonded carbon has two different substituents.

• Cis (Z): Same groups on same side
• Trans (E): Same groups on opposite sides

Reactions of Alkenes:

1. Addition reactions (electrophilic addition):

   • H₂/H₂SO₄ (catalyst): Hydrogenation → alkane
   • HX (X = Cl, Br, I): Hydrohalogenation → haloalkane
     • Markovnikov’s rule: H adds to carbon with MORE hydrogens; X adds to carbon with FEWER hydrogens
   • H₂O/H⁺: Hydration → alcohol
   • X₂ (Cl₂, Br₂): Halogenation → vicinal dihalide
   • Br₂ in CCl₄: Decolorizes orange bromine (test for unsaturation)

2. Polymerization: n CH₂=CH₂ → (–CH₂–CH₂–)ₙ (polyethene)

3. Combustion: CₙH₂ₙ + (3n/2)O₂ → nCO₂ + nH₂O

Example — Reaction of propene with HBr: CH₃–CH=CH₂ + HBr → CH₃–CH(Br)–CH₃ (2-bromopropane, major product — Markovnikov addition)

3.2 Alcohols (–OH Hydroxyl Group)

Classification:

• Primary (1°): –OH attached to C with one alkyl group (R–CH₂OH)
• Secondary (2°): –OH attached to C with two alkyl groups (R₂CHOH)
• Tertiary (3°): –OH attached to C with three alkyl groups (R₃COH)

Naming: Root + -ol + position number

• methanol, ethanol, propan-1-ol, propan-2-ol, ethane-1,2-diol (glycol)

Reactions:

1. Combustion: Complete combustion produces CO₂ + H₂O
2. Oxidation (by acidified K₂Cr₂O₇ or KMnO₄):
   • 1° alcohol → aldehyde → carboxylic acid
   • 2° alcohol → ketone
   • 3° alcohol → NO oxidation (no reaction under mild conditions)
3. Esterification: R–OH + R’–COOH → R’–COOR + H₂O (Fischer esterification)
4. Reaction with PCl₅: R–OH + PCl₅ → R–Cl + POCl₃ + HCl
5. Dehydration: R–CH₂–CH₂OH → (conc. H₂SO₄, 170°C) → R–CH=CH₂ + H₂O (elimination)

Hydrogen bonding: Alcohols have higher boiling points than alkanes of similar MW due to H-bonding between –OH groups.

Example — Oxidation of ethanol: CH₃CH₂OH → (K₂Cr₂O₇/H⁺) → CH₃CHO → (further oxidation) → CH₃COOH

3.3 Aldehydes and Ketones (C=O Carbonyl Group)

Aldehyde: –CHO (carbonyl at end of chain), suffix -al Ketone: –CO– (carbonyl in middle), suffix -one

Naming:

• Aldehydes: methanal (formaldehyde), ethanal, propanal
• Ketones: propanone (acetone), butanone, pentan-2-one

Reactions:

1. Nucleophilic addition (primary reaction type):

   • With H₂/Ni: → alcohol (1° from aldehyde, 2° from ketone)
   • With 2,4-DNP reagent: → 2,4-dinitrophenylhydrazone (orange precipitate) — used to detect/identify carbonyls
   • With NaHSO₃: → bisulfite addition compound (white crystalline precipitate) — useful for purification
   • With R–MgX (Grignard): → alcohol after acidic workup

2. Oxidation:

   • Aldehydes: oxidize to carboxylic acids (Fehling’s solution: blue → red Cu₂O; Tollens’ reagent: Ag⁺ → Ag mirror)
   • Ketones: do NOT oxidize under mild conditions (distinguishes aldehydes from ketones)

3. Reduction (NaBH₄ or H₂/Ni): → alcohol

4. Haloform reaction (methyl ketones only): CH₃–CO–R + 3I₂ + 4NaOH → CHI₃ (yellow precipitate) + R–COONa + 3NaI + 3H₂O

Tollen’s Test (distinguishes aldehydes from ketones): Aldehyde + Ag(NH₃)₂⁺ → Ag⁰ (silver mirror) + carboxylic acid Ketone: No reaction

Fehling’s Test: Aldehyde + Cu²⁺ (blue) → Cu₂O (red precipitate) Ketone: No reaction

3.4 Carboxylic Acids (–COOH)

Naming: Root + -oic acid

• methanoic acid (formic acid), ethanoic acid (acetic acid), benzoic acid

Physical properties:

• H-bonding → relatively high boiling points
• First four members are water-soluble
• Weak acids (pKa ~ 4.76 for acetic acid)

Reactions:

1. Neutralization: R–COOH + NaOH → R–COONa + H₂O
2. Esterification: R–COOH + R’–OH → R–COOR’ + H₂O
3. Reduction: R–COOH → (LiAlH₄) → R–CH₂OH (primary alcohol)
4. Decarboxylation: Removal of CO₂ from carboxyl group (requires heating with soda lime: R–COONa + NaOH → R–H + Na₂CO₃)

Derivatives of carboxylic acids:

• Acid chloride: R–COCl (very reactive, hydrolyzes to acid)
• Acid anhydride: R–CO–O–CO–R (e.g., acetic anhydride)
• Ester: R–COOR’ (fragrant, fruity odors)

3.5 Amines (–NH₂)

Classification:

• Primary (1°): R–NH₂ (one alkyl group attached to N)
• Secondary (2°): R₂NH (two alkyl groups attached to N)
• Tertiary (3°): R₃N (three alkyl groups attached to N)

Naming: Alkane + -amine (or prefix amino-)

• methylamine (CH₃NH₂), dimethylamine, trimethylamine, ethanamine

Properties:

• Ammonia-like odor
• Basic (nitrogen lone pair accepts H⁺)
• Lower MW amines are water-soluble
• Can form H-bonds (1° and 2° amines)

Reactions:

1. As bases: R–NH₂ + HCl → R–NH₃⁺Cl⁻
2. Acylation: R–NH₂ + R’–COCl → R–NH–CO–R’ + HCl (forms amide)
3. With nitrous acid (HNO₂):
   • 1° amine → diazonium salt → (unstable) → N₂ gas + carbocation → products
   • 2° amine → N-nitrosoamine (yellow oil)
   • 3° amine → no reaction

Example — Reaction of ethylamine with acetic anhydride: CH₃CH₂NH₂ + (CH₃CO)₂O → CH₃CH₂NHCOCH₃ + CH₃COOH (forms N-ethylacetamide)

3.6 Esters (–COO–)

Naming: Alkyl alkanoate

• methyl ethanoate (CH₃COOCH₃), ethyl ethanoate (CH₃COOCH₂CH₃)

Formation (Fischer esterification): R–COOH + R’–OH ⇌ R–COOR’ + H₂O (catalyzed by acid, reversible)

Hydrolysis:

• Acidic hydrolysis: R–COOR’ + H₂O → R–COOH + R’–OH (reverse of esterification)
• Alkaline hydrolysis (saponification): R–COOR’ + NaOH → R–COONa + R’–OH

Distinguishing esters: Esters do not give positive results with Tollens’ or Fehling’s tests (no aldehyde group). They can be hydrolyzed to carboxylate salt under basic conditions.

4. General Reaction Patterns

Addition vs Substitution

• Addition: Unsaturated compounds (alkenes, alkynes) add atoms across multiple bonds
• Substitution: One atom/group replaces another (alkanes, aromatic compounds)

Oxidation and Reduction

• Oxidation (gain O or lose H):
  • 1° alcohol → aldehyde → carboxylic acid
  • 2° alcohol → ketone
  • Aldehyde → carboxylic acid
• Reduction (gain H or lose O):
  • Aldehyde → 1° alcohol
  • Ketone → 2° alcohol
  • Carboxylic acid → alcohol (needs LiAlH₄)

Elimination

Removal of small molecule (H₂O, HX) to form double/triple bonds.

• Alcohol dehydration: conc. H₂SO₄, 170°C
• Haloalkane dehydrohalogenation: alcoholic KOH, heat

5. Exam-Style Questions & Tips

Common exam question patterns at Makerere:

1. “Identify the functional groups in compound [X]”
2. “Name the following compounds and state the functional group(s)”
3. “Write the structural formula of [compound name]”
4. “Predict the major product of [reaction] and name it”
5. “How would you distinguish between [compound A] and [compound B]?”
6. “Write the mechanism of [reaction type]”
7. “Explain why [property difference] between two compounds”

⚡ Exam tips:

• Always identify the functional group first before predicting reactions
• In naming, the suffix tells you the priority functional group (carboxylic acid > aldehyde > ketone > alcohol > amine)
• For distinguishing tests: Fehling’s/Tollens’ distinguishes aldehydes; bromine water distinguishes alkenes; NaHCO₃ distinguishes carboxylic acids (CO₂ evolution)
• When drawing mechanisms, show curly arrows clearly — arrow starts from electron pair or bond, points to where new bond forms or breaks
• Know the characteristic colors/smells: aldehydes can smell pungent, esters are fruity, amines are fishy/ammonia-like

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Reaction Mechanisms — Detailed

Electrophilic Addition to Alkenes

The π electrons of the C=C double bond are nucleophilic and attack electrophiles (E⁺).

Mechanism of HBr addition to propene:

1. HBr dissociates → H⁺ (electrophile) + Br⁻ (nucleophile)
2. π electrons attack H⁺ → carbocation intermediate (more stable = secondary carbocation at C-2) CH₃–CH⁺–CH₃ (secondary) + Br⁻ → CH₃–CH(Br)–CH₃
3. Carbocation rearrangement is possible if it forms a more stable intermediate

Markovnikov’s Rule (electrophilic addition to unsymmetrical alkenes): The electrophile adds to the carbon with more hydrogen atoms, and the nucleophile adds to the carbon with fewer hydrogen atoms.

Nucleophilic Addition to Carbonyls

The carbonyl carbon (δ+) is electrophilic due to oxygen’s electronegativity. Nucleophiles attack.

Mechanism of aldehyde + HCN:

1. CN⁻ attacks carbonyl carbon → alkoxide intermediate
2. H⁺ from acid protonates the alkoxide → cyanohydrin R–CHO + HCN → R–CH(OH)–CN

Mechanism of aldehyde + Grignard reagent:

1. R’⁻ (nucleophilic carbon of Grignard) attacks carbonyl carbon
2. Acid workup protonates the alkoxide → alcohol R–CHO + R’MgX → R–CH(OMgX)R’ → (H₃O⁺) → R–CH(OH)R’

Nucleophilic Substitution (SN1 and SN2)

SN2 Mechanism (bimolecular, concerted):

• One-step mechanism
• Backside attack; inversion of configuration (Walden inversion)
• Rate = k[ substrate ][ nucleophile ]
• Best for: methyl halides, primary halides, polar aprotic solvents
• Good nucleophiles favored

SN1 Mechanism (unimolecular, stepwise):

1. Slow ionization to form carbocation (rate-determining)
2. Fast attack by nucleophile

• Rate = k[ substrate ] only
• Best for: tertiary halides, stable carbocations
• Carbocation can rearrange

SN1 vs SN2:

Factor	SN1	SN2
Kinetics	Unimolecular	Bimolecular
Stereochemistry	Racemic mixture	Inversion
Substrate	3° > 2° >> 1°	1° > 2° > 3°
Nucleophile	Weak	Strong
Solvent	Polar protic	Polar aprotic

Elimination (E1 and E2)

E2 (bimolecular, concerted):

• One-step, anti-periplanar elimination
• Base abstracts H while C–X bond breaks simultaneously
• Hofmann product favored with bulky bases
• Rate = k[ substrate ][ base ]

E1 (unimolecular, stepwise):

1. Ionization to carbocation
2. Base removes H⁺ from adjacent carbon

• Zaitsev product favored (more substituted alkene)
• Rate = k[ substrate ]

Elimination vs Substitution

• 3° substrate + strong base → E2 (elimination)
• 1° substrate + strong base → SN2 (substitution)
• 3° substrate + weak base → E1 or SN1 (mix)
• Polar aprotic solvent favors SN2
• Bulkier base (t-BuOK) favors elimination

Polarity and Solubility

”Like dissolves like”

• Polar compounds dissolve in polar solvents (water)
• Non-polar compounds dissolve in non-polar solvents (hexane)
• Organic compounds with –OH, –NH₂, –COOH can H-bond with water

Effect of Functional Group on Physical Properties

Property	Trend
Boiling point	↑ with ↑ MW, ↑ polarity, ↑ H-bonding
Water solubility	↑ with ↑ polarity, ↑ H-bonding ability
Melting point	↑ with ↑ symmetry, ↑ H-bonding

Biomolecules — Quick Overview

Carbohydrates

• Monosaccharides: glucose (C₆H₁₂O₆), fructose — simple sugars
• Disaccharides: sucrose (glucose + fructose), lactose (glucose + galactose)
• Polysaccharides: starch, cellulose, glycogen — polymers of glucose

Amino Acids and Proteins

• Amino acids: H₂N–CH(R)–COOH — building blocks of proteins
• 20 standard amino acids (proteinogenic)
• Peptide bond: –CO–NH– links amino acids in proteins

Lipids

• Fats and oils: triglycerides (ester of glycerol + 3 fatty acids)
• Unsaturated fats: contain C=C double bonds (liquid at room temperature)
• Saturated fats: no C=C bonds (solid at room temperature)

Nucleic Acids

• DNA: deoxyribonucleic acid — double helix, genetic information
• RNA: ribonucleic acid — single stranded, involved in protein synthesis
• Nucleotides: phosphate + sugar + nitrogenous base

Practice Problems

Q1: Identify all functional groups in: (a) CH₃CH₂COOCH₃ (b) C₆H₅NH₂ (c) CH₃CH(OH)CH₂CHO (d) HOOC–CH₂–CH(NH₂)–COOH

Q2: Write the structure of butanoic acid and explain why it has a higher boiling point than diethyl ether (both C₄H₈O).

Q3: Describe chemical tests to distinguish between: (a) propanal and propanone (b) ethanoic acid and ethanol (c) ethene and ethane

Q4: 1-bromobutane undergoes substitution with NaOH. Predict the major product and give your reasoning about the mechanism.

Q5: Draw the structure of the polymer formed from propene (polypropene). What type of polymerization is this?

Common Mistakes to Avoid

1. Forgetting the distinction between primary, secondary, and tertiary alcohols: This determines oxidation products — always classify before writing reactions.
2. Drawing the wrong product for Markovnikov addition: The H goes to the carbon with MORE hydrogens already; the halogen goes to the carbon with FEWER hydrogens.
3. Confusing functional group tests: Tollens’ (silver mirror) and Fehling’s (red precipitate) are for aldehydes only — ketones don’t give these tests.
4. Forgetting that esterification is reversible: Le Chatelier’s principle applies — excess alcohol or acid, or removal of water, drives the reaction forward.
5. Naming with wrong position numbers: Always number from the end that gives the functional group the lowest possible number.

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