Structure and Synthesis of Alcohols (Chapter 10): Study Notes

Chapter 10: Structure and Synthesis of Alcohols

Introduction to Alcohols

Alcohols are organic compounds containing a hydroxyl group (-OH) bonded to a saturated carbon atom. Their structure and reactivity are central to organic synthesis and functional group transformations.

• General Formula: $R-OH$

• Example: Ethanol ($CH_3CH_2OH$)

• Structural Similarity: Alcohols are structurally similar to water ($H_2O$) and can be classified as primary (1°), secondary (2°), or tertiary (3°) based on the number of alkyl groups attached to the carbon bearing the hydroxyl group.

Naming Alcohols

The IUPAC system is used to name alcohols, prioritizing the longest carbon chain containing the hydroxyl group.

1. Find the longest carbon chain containing the alcohol group.

2. Number the chain so that the carbon with the -OH group gets the lowest possible number.

3. Name and number substituents, and determine stereochemistry (R/S, E/Z) if necessary.

4. Replace the terminal '-e' of the alkane name with '-ol'.

Example: 2-propylhex-3-en-5-yn-1-ol (with stereochemistry: (S,E))

Phenols and Benzene Diols

• Phenol: An aromatic alcohol where the -OH group is directly attached to a benzene ring.

• Benzene Diols: Compounds with two hydroxyl groups on a benzene ring, e.g., benzene-1,3-diol.

• In aromatic systems, the alcohol is assumed to be at the 1-position unless otherwise specified.

Hydrogen Bonding in Alcohols

Alcohols can form hydrogen bonds due to the presence of the -OH group, which significantly increases their boiling points compared to analogous alkanes and ethers.

• Hydrogen Bonding: $R-OH ext{---} O-HR$

• Responsible for relatively high boiling points and solubility in water.

Acidity of Alcohols and Phenols

• Alcohols: Typical pKa ≈ 16

• Phenols: More acidic, pKa ≈ 10, due to resonance stabilization of the phenoxide ion.

Resonance Stabilization of Phenoxide:

• Phenol loses a proton to form phenoxide, which is stabilized by delocalization of the negative charge over the aromatic ring.

Williamson Ether Synthesis

The Williamson ether synthesis is a method for preparing ethers from alcohols via deprotonation and nucleophilic substitution.

• General Reaction:

$R-OH + NaH \rightarrow R-O^-Na^+ \xrightarrow{R'-X} R-O-R' + NaX$

• Alcohol is deprotonated by sodium hydride (NaH) to form an alkoxide, which then reacts with a primary alkyl halide ($R'-X$) to form an ether.

Synthesis of Alcohols

Alcohols can be synthesized by various methods, including:

1. Hydration of Alkenes: $RCH=CH_2 + H_2O \xrightarrow{H^+} RCH(OH)CH_3$

2. Hydroboration-Oxidation: $RCH=CH_2 \xrightarrow{1. BH_3, 2. H_2O_2/OH^-} RCH_2CH_2OH$

3. Reduction of Carbonyl Compounds: Aldehydes and ketones can be reduced to primary and secondary alcohols, respectively.

4. Organometallic Addition: Grignard reagents ($R-MgX$) or organolithium reagents ($R-Li$) add to carbonyl compounds to form alcohols after aqueous workup.

Organometallic Reagents

• Organolithium: $R-Li$ (Electronegativity difference: Li = 1.0, C = 2.5)

• Grignard Reagents: $R-MgX$ (X = Cl, Br, I)

• Both are strong nucleophiles and bases, used to form carbon-carbon bonds.

Synthesis of Grignard Reagents

• Formed by reaction of alkyl or aryl halides with magnesium metal in dry ether:

$R-X + Mg \xrightarrow{Et_2O} R-MgX$

• Works best with iodides and bromides; less effective with chlorides and not with fluorides.

• Also applicable to aryl halides (aromatic systems).

Reactivity of Grignard Reagents

• Grignard reagents react with carbonyl compounds to form alcohols after hydrolysis.

• They do not undergo $S_N2$ reactions with alkyl halides.

• They are very strong bases and react with water, alcohols, and amines faster than with carbonyls.

Grignard Reactions with Carbonyl Compounds

• Aldehydes and Ketones: Grignard reagents add to the carbonyl carbon, forming a new C–C bond and, after hydrolysis, a secondary or tertiary alcohol.

• Esters: Require two equivalents of Grignard reagent to yield tertiary alcohols.

• Acid Chlorides: Even more reactive than esters; also yield tertiary alcohols.

$R-MgX + R'COOR'' \xrightarrow{2 eq.} R'COR + R''OMgX \xrightarrow{H_2O} R'COR + R''OH$

Acidity and Basicity Comparison Table

Compound	pKa	Acidity
Alkyne ($HC\equiv CH$)	~25	Less acidic
Alcohol ($R-OH$)	~16	More acidic
Phenol	~10	Most acidic
Alkane ($CH_4$)	~50	Least acidic

Deuterium Exchange Using Grignard Reagents

• Grignard reagents can be used to introduce deuterium ($D_2O$) in place of a halide or proton.

• Example: $R-MgX + D_2O \rightarrow R-D + MgXOD$

Reduction of Carbonyl Compounds

• Sodium Borohydride ($NaBH_4$): Reduces aldehydes and ketones to alcohols; does not reduce esters.

• Lithium Aluminum Hydride ($LiAlH_4$): Stronger reducing agent; reduces aldehydes, ketones, and esters to alcohols.

General Mechanism:

• Hydride addition to the carbonyl carbon, followed by protonation to yield the alcohol.

Summary Table: Reducing Agents and Their Reactivity

Reducing Agent	Reduces Aldehydes/Ketones?	Reduces Esters?
$NaBH_4$	Yes	No
$LiAlH_4$	Yes	Yes

Key Mechanisms

• Hydride Addition: Nucleophilic attack of hydride on carbonyl carbon, forming a tetrahedral intermediate, followed by protonation.

• Grignard Addition: Nucleophilic attack of $R^-$ from $R-MgX$ on carbonyl carbon, followed by aqueous workup to yield alcohol.

Additional Info:

• Grignard reagents are incompatible with protic solvents (e.g., water, alcohols) due to their strong basicity.

• Any C–C bond to an alcohol carbon can be a synthetic disconnection in retrosynthetic analysis.

• Lithium alkynides ($R-C\equiv C-Li$) can perform $S_N2$ reactions with primary alkyl halides only.