Alkanes are saturated hydrocarbons (contain only C-C single bonds) with the general formula CₙH₂ₙ₊₂.
Formation of Alkanes
Catalytic Hydrogenation (Sabatier-Senderens reaction): Alkenes or alkynes react with hydrogen gas (H₂) in the presence of a catalyst (Ni, Pt, or Pd) to form alkanes.
Reaction: R-CH=CH-R' + H₂ --(Ni/Pt/Pd)--> R-CH₂-CH₂-R'
Wurtz Reaction: Alkyl halides react with sodium (Na) in dry ether to form a symmetrical alkane with double the number of carbon atoms.
Reaction: 2 R-X + 2 Na --(Dry Ether)--> R-R + 2 NaX
Limitation: Only good for making symmetrical alkanes (R-R). If two different alkyl halides are used (R-X and R'-X), a mixture of products (R-R, R'-R', and R-R') is formed, which is hard to separate.
Corey-House Synthesis: A superior method for making symmetrical, unsymmetrical, and substituted alkanes.
Steps:
Alkyl halide (R-X) reacts with Lithium (Li) → R-Li (Alkyllithium)
Advantage: Can form unsymmetrical alkanes (R-R') efficiently. R' should ideally be methyl, 1°, or 2° cyclic.
Reactions of Alkanes: Free Radical Substitution
Alkanes are generally unreactive. Their most characteristic reaction is free radical substitution, like halogenation.
Halogenation: Reaction with Cl₂ or Br₂ in the presence of UV light or high temperature.
Mechanism (e.g., Chlorination of Methane):
Initiation: Homolytic fission of the halogen.
Cl-Cl --(UV light)--> 2 Cl• (Chlorine free radicals)
Propagation: The radical attacks the alkane, creating a new radical, which continues the chain.
Cl• + CH₄ → •CH₃ + HCl
•CH₃ + Cl₂ → CH₃Cl + Cl•
Termination: Two radicals combine to end the chain.
Cl• + Cl• → Cl₂
•CH₃ + •CH₃ → CH₃-CH₃
Cl• + •CH₃ → CH₃Cl
Relative Reactivity and Selectivity:
Reactivity of Halogens: F₂ > Cl₂ > Br₂ > I₂ (Fluorine is explosive, Iodine is unreactive).
Reactivity of H atoms: The ease of abstracting a hydrogen atom follows the stability of the free radical formed: 3° H > 2° H > 1° H.
Selectivity: Bromination (Br₂) is much *more selective* than chlorination (Cl₂). Bromine will almost exclusively attack the most stable 3° C-H bond, whereas chlorine gives a mixture of products.
Alkenes: Formation and Elimination Mechanisms
Alkenes are unsaturated hydrocarbons (contain at least one C=C double bond) with the general formula CₙH₂ₙ.
Alkenes are typically formed by elimination reactions.
Types of Elimination Reactions
Dehydration of Alcohols: Removal of water (H₂O) from an alcohol using a strong acid (e.g., conc. H₂SO₄ or H₃PO₄) and heat.
Dehydrohalogenation of Alkyl Halides: Removal of a hydrogen halide (H-X) from an alkyl halide using a strong base (e.t., alcoholic KOH).
Saytzeff (Zaitsev) vs. Hoffmann Elimination
Saytzeff's Rule: In an elimination reaction, the more substituted (more stable) alkene is the major product. This is the thermodynamic product.
Favored by: Small, strong bases (e.g., KOH, NaOEt) and 2°/3° substrates.
Hoffmann Elimination: The less substituted (less stable) alkene is the major product. This is the kinetic product.
Favored by: Bulky, strong bases (e.g., (CH₃)₃CO⁻K⁺, or potassium tert-butoxide) or substrates with a poor leaving group (like -F).
Mechanisms of Elimination
There are three main mechanisms for elimination reactions.
Mechanism
Description
Kinetics
Substrate
Base
E2 (Elimination Bimolecular)
A one-step (concerted) reaction. The base removes a proton (H⁺) at the same time the leaving group (X⁻) departs. Requires an anti-periplanar geometry (H and X must be 180° apart).
A two-step reaction.
1. Leaving group departs, forming a carbocation (rate-determining step).
2. Base removes a proton from the adjacent carbon.
Rate = k[Substrate] (Unimolecular, 1st order)
Favored by 3° > 2° (due to carbocation stability).
Can occur with weak bases (e.g., H₂O, ROH). Often competes with SN1.
E1cb (Elimination Unimolecular Conjugate Base)
A two-step reaction.
1. Base removes a proton to form a carbanion (conjugate base).
2. Leaving group departs from the carbanion.
Rate = k[Substrate][Base] (Usually)
Requires a poor leaving group (e.g., -F, -OH) AND an acidic proton (e.g., alpha to a C=O group).
Requires a strong base.
Alkenes: Reactions
The C=C double bond is an electron-rich (nucleophilic) center, so its characteristic reaction is electrophilic addition.
Electrophilic Additions
Addition of H-X (Markownikoff's Rule):
Markownikoff's Rule: When an unsymmetrical reagent (like H-X) adds to an unsymmetrical alkene, the negative part of the reagent (X⁻) adds to the carbon atom that has *fewer* hydrogen atoms (i.e., the more substituted carbon).
Reason: The reaction proceeds via the more stable carbocation intermediate.
Reaction: CH₃-CH=CH₂ + HBr → CH₃-CH(Br)-CH₃ (2-bromopropane, major product)
Anti-Markownikoff Addition (Peroxide Effect):
In the presence of peroxides (R-O-O-R), the addition of HBr (only HBr!) follows a free-radical mechanism and gives the Anti-Markownikoff product.
Anti-Markownikoff Rule: The negative part (Br) adds to the carbon with *more* hydrogen atoms.
Reason: The reaction proceeds via the more stable free radical intermediate.
Reaction: CH₃-CH=CH₂ + HBr --(Peroxide)--> CH₃-CH₂-CH₂Br (1-bromopropane, major product)
Other Important Reactions
Hydroboration-Oxidation: A two-step reaction that achieves Anti-Markownikoff, syn-addition of water (H and OH).
Step 1 (Hydroboration): Alkene + BH₃ (Borane). The Boron adds to the less substituted C.
Step 2 (Oxidation): H₂O₂, NaOH. The B is replaced by an -OH group.
Net Result: CH₃-CH=CH₂ → CH₃-CH₂-CH₂-OH (Propan-1-ol).
Oxymercuration-Demercuration: A two-step reaction that achieves Markownikoff addition of water, with no rearrangement.
Step 1 (Oxymercuration): Alkene + Hg(OAc)₂, H₂O.
Step 2 (Demercuration): NaBH₄.
Net Result: CH₃-CH=CH₂ → CH₃-CH(OH)-CH₃ (Propan-2-ol).
Exam Tip: Use acid-catalyzed hydration for Markownikoff addition *if* rearrangement is not possible. Use Oxymercuration-Demercuration for Markownikoff addition to *prevent* rearrangement. Use Hydroboration-Oxidation for Anti-Markownikoff addition.
Ozonolysis: Cleavage of the C=C double bond by ozone (O₃).
Step 1: Alkene + O₃ → Ozonide.
Step 2 (Workup):
Reductive Workup (Zn/H₂O or (CH₃)₂S): Cleaves the bond and gives aldehydes and/or ketones.
R-CH=CH-R' → R-CHO + R'-CHO
Oxidative Workup (H₂O₂): Cleaves the bond and gives carboxylic acids and/or ketones. (Aldehydes are oxidized further).
R-CH=CH-R' → R-COOH + R'-COOH
Application: Used to determine the location of a double bond in an unknown molecule.
Diels-Alder Reaction: A [4+2] cycloaddition reaction between a conjugated diene (4 π-electrons) and a dienophile (2 π-electrons) to form a six-membered ring.
Example: Buta-1,3-diene + Ethene → Cyclohexene.
Note: The diene must be in the s-cis conformation. The reaction is favored by electron-donating groups (EDGs) on the diene and electron-withdrawing groups (EWGs) on the dienophile.
Alkynes
Alkynes are unsaturated hydrocarbons (contain at least one C≡C triple bond) with the general formula CₙH₂ₙ₋₂.
Formation of Alkynes
Alkynes are formed by a double dehydrohalogenation of a geminal or vicinal dihalide. This requires a very strong base, such as sodium amide (NaNH₂) in liquid ammonia, or molten KOH.
Geminal dihalide: Halogens on the same carbon.
R-CH₂-CH(Br)₂ + 2 NaNH₂ → R-C≡C-H + 2 NaBr + 2 NH₃
Acidity of Terminal Alkynes
The hydrogen attached to a terminal alkyne (R-C≡C-H) is weakly acidic (pKa ≈ 25).
Reason: The carbon atom is sp-hybridized. It has 50% s-character, making it highly electronegative. It pulls the C-H bond electrons strongly, allowing the H to be removed as H⁺ by a strong base (like NaNH₂).
This acidity allows for the formation of acetylide anions (R-C≡C:⁻), which are excellent nucleophiles.
Reactions of Alkynes
Electrophilic Addition: Similar to alkenes, but can happen twice. Markownikoff's rule applies.
R-C≡C-H + HBr → R-C(Br)=CH₂ (Markownikoff product)
R-C(Br)=CH₂ + HBr → R-C(Br)₂-CH₃ (Geminal dihalide)
Nucleophilic Addition: Unlike alkenes, alkynes can undergo nucleophilic addition because the sp-carbon is electron-withdrawing, making the triple bond susceptible to attack by nucleophiles. (e.g., addition of methanol).
Hydration (Addition of Water):
Markownikoff Hydration: Uses H₂SO₄ and HgSO₄ (catalyst).
R-C≡C-H + H₂O --(HgSO₄/H₂SO₄)--> [R-C(OH)=CH₂] (an enol, unstable)
The enol immediately tautomerizes to the more stable ketone.
[R-C(OH)=CH₂] ⇌ R-C(=O)-CH₃ (a methyl ketone)
Alkylation of Terminal Alkynes: This is an SN2 reaction using the acetylide anion.
Step 1: R-C≡C-H + NaNH₂ → R-C≡C:⁻Na⁺ (Sodium acetylide)
Step 2: R-C≡C:⁻Na⁺ + R'-X → R-C≡C-R' + NaX
Note: R'-X must be a methyl or 1° alkyl halide for this to work. 2° and 3° halides will undergo E2 elimination instead.