CHEMCA
EXAM MASTER FORMULA SHEET
Organic Chemistry: Alcohols
1. Physical Properties & Hydrogen Bonding
Alcohols ($\ce{R-OH}$) possess significantly higher boiling points than hydrocarbons, ethers, or haloalkanes of comparable molecular mass due to Intermolecular Hydrogen Bonding.
- • B.P. increases with increase in carbon chain length (increased van der Waals forces).
- • Among isomeric alcohols, B.P. decreases with branching (surface area decreases).
- • Lower alcohols (up to 3 carbons) are highly miscible in water due to H-bonding.
- • Solubility decreases as the size of the hydrophobic alkyl group ($R$) increases.
Solubility $\propto \frac{1}{\text{Molecular Mass}}$
2. Methods of Preparation
Reagent: $\ce{dil. H2SO4}$ or $\ce{H2O / H+}$
Markovnikov Addition
*Rearrangement of carbocation possible!*
Reagent: 1. $\ce{Hg(OAc)2, H2O}$ 2. $\ce{NaBH4}$
Markovnikov Addition
*NO rearrangement (cyclic intermediate).*
Reagent: 1. $\ce{B2H6, THF}$ 2. $\ce{H2O2 / OH-}$
Anti-Markovnikov Addition
*SYN addition. NO rearrangement.*
- • $\ce{R-CHO ->[LiAlH4 \text{ or } NaBH4] R-CH2-OH}$ ($1^\circ$)
- • $\ce{R-CO-R' ->[LiAlH4 \text{ or } NaBH4] R-CH(OH)-R'}$ ($2^\circ$)
- • $\ce{R-COOH ->[LiAlH4] R-CH2-OH}$ (Strong! $NaBH_4$ fails)
- • $\ce{R-COOR' ->[LiAlH4] R-CH2-OH + R'-OH}$
Nucleophilic addition of Grignard reagent to carbonyls followed by hydrolysis.
- 1. $\ce{HCHO + RMgX ->[H3O+] R-CH2-OH}$ ($1^\circ$)
- 2. $\ce{R'-CHO + RMgX ->[H3O+] R'-CH(OH)-R}$ ($2^\circ$)
- 3. $\ce{R'-CO-R'' + RMgX ->[H3O+] R'R''R-C-OH}$ ($3^\circ$)
3. Chemical Reactions
Alcohols are weak Brønsted acids. Alkyl groups ($+I$ effect) increase electron density on Oxygen, making the $O-H$ bond stronger.
Reaction with active metals (Na, K) yields $H_2$ gas and alkoxides.
Reaction with Carboxylic Acids, Acid Chlorides, or Anhydrides.
*Isotope labelling proves the Oxygen in water comes from the Acid, NOT the alcohol!*
Reactivity order of alcohols: $3^\circ > 2^\circ > 1^\circ$ (Proceeds via Carbocation / $S_N1$ for $2^\circ$ and $3^\circ$).
$\ce{R-OH + PCl5 -> R-Cl + POCl3 + HCl}$
Preferred because byproducts are escapable gases, leaving pure alkyl chloride.
Heating with concentrated $\ce{H2SO4}$ or $\ce{H3PO4}$ at high temperatures.
- Follows Saytzeff Rule (highly substituted alkene is major).
- Proceeds via Carbocation intermediate $\to$ Rearrangements are common!
- Ease of dehydration: $3^\circ > 2^\circ > 1^\circ$. ($3^\circ$ needs only 20% $H_2SO_4$ at 358K).
4. Oxidation & Distinguishing Tests
| Alcohol Type | Strong Oxidant ($KMnO_4, K_2Cr_2O_7 / H^+$) | Mild Oxidant (PCC / PDC) | Heated Cu ($573 K$) |
|---|---|---|---|
| $1^\circ$ (Primary) | Aldehyde $\xrightarrow{\text{fast}}$ Carboxylic Acid | Aldehyde (Stops here) | Aldehyde |
| $2^\circ$ (Secondary) | Ketone (Resists further oxidation) | Ketone | Ketone |
| $3^\circ$ (Tertiary) | Resistant. Drastic conditions cleave C-C bonds to give mixture of acids. | No Reaction | Alkene (Dehydration!) |
Reagent: Anhydrous $ZnCl_2 + \text{Conc. } HCl$. Distinguishes based on reactivity towards $S_N1$.
- • $3^\circ$ Alcohol: Immediate cloudiness/turbidity.
- • $2^\circ$ Alcohol: Turbidity appears after 5-10 minutes.
- • $1^\circ$ Alcohol: No turbidity at room temperature (only on heating).
Sequence: $P/I_2 \rightarrow AgNO_2 \rightarrow HNO_2 \rightarrow NaOH$
Blood Red
Blue
Colorless
Used to identify the presence of the $\ce{CH3-CH(OH)-}$ group (e.g., Ethanol, Propan-2-ol, Butan-2-ol) or the $\ce{CH3-C(=O)-}$ group. Methanol and $1^\circ$ alcohols (except ethanol) fail this test.
$\ce{CHI3}$ (Iodoform) forms a characteristic YELLOW precipitate.
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