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Organic Chemistry II - Alcohols and Ethers

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    Study Guide Organic Chemistry II – Alcohols and Ethers 1. Reactions of Alcohols Alcohols are very important organic compounds because they can be converted into many other useful substances. This wide range of reactions is due to the presence of the hydroxyl ( – OH) group . In this chapter, we will study how alcohols react and what products are formed under different conditions. What Can Alcohols Be Converted Into? Alcohols can be converted into: • Metal salts (alkoxides) • Alkyl halides • Esters • Aldehydes • Ketones • Carboxylic acids Each type of reaction is discussed below in a clear and step - by - step manner. 1. Metal Salt (Alkoxide) Formation Acidic nature of alcohols Alcohols behave as weak acids . They are only slightly weaker acids than water. • (K a ) of alcohols ≈ 1 × 10 ⁻ ¹⁶ • (K a ) of water ≈ 1 × 10 ⁻ ¹⁵

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    Study Guide Reaction with sodium metal When ethanol reacts with sodium metal, it forms sodium ethoxide and releases hydrogen gas . This reaction is very similar to the reaction between sodium metal and water, but the reaction with water occurs faster because water is slightly more acidic. Similar reactions also occur with potassium metal . Effect of structure on acidity • Acidity of alcohols decreases in the order: Primary > Secondary > Tertiary This decrease happens because: 1. Alkyl groups increase electron density on oxygen, making proton removal difficult. 2 . Bulky alkyl groups reduce solvation of the alkoxide ion. Both effects increase the energy needed to remove a proton. Basicity of alkoxide ions • Basicity increases from primary to tertiary alkoxides . • A weaker acid always has a stronger conjugate base . 2. Formation of Alkyl Halides from Alcohols Alcohols can be converted into alkyl halides by reacting with hydrogen halides (HCl, HBr, HI) .

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    Study Guide Reaction mechanism • Primary alcohols mainly undergo S N 2 reactions. • Tertiary alcohols mainly undergo S N 1 reactions. Heat is usually required to carry out these reactions. Key point: The structure of the alcohol decides whether the reaction follows S N 1 or S N 2 . 3. Alkyl Chloride Formation Using Thionyl Chloride (SOCl ₂ ) A more efficient method for preparing alkyl chlorides from alcohols uses thionyl chloride (SOCl ₂ ) . Advantages of this method • Reaction is fast • Fewer side products • By - products (SO ₂ and HCl) are gases and escape easily Mechanism (simplified) 1. The alcohol first reacts with SOCl ₂ to form an inorganic ester . 2. Chloride ion attacks the carbon atom by an S N 2 mechanism . 3. Products formed are alkyl chloride, SO ₂ , and HCl .

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    Study Guide Stereochemical result Since the reaction mainly follows an S N 2 pathway, an optically active alcohol gives a product with inverted configuration . 4. Formation of Alkyl Bromides and Alkyl Iodides Alkyl bromides • Thionyl bromide is unstable. • Therefore, phosphorus tribromide (PBr ₃ ) is used. Mechanism: 1. Alcohol reacts with PBr ₃ to form a phosphorus ester.

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    Study Guide 2. Bromide ion replaces the ester group via S N 1 or S N 2 substitution. Alkyl iodides • Prepared in a similar way using phosphorus triiodide (PI ₃ ) . 5. Ester Formation (Fischer Esterification) What are esters? Esters are formed when alcohols react with carboxylic acids . • Ester functional group: – COOR Fischer esterification process • Alcohol + carboxylic acid • Acid catalyst (H ⁺ ) • Heat is applied This reaction is reversible . Improving yield

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    Study Guide Because the reaction is an equilibrium reaction, a higher yield of ester is obtained by removing water or ester as it forms . Mechanism overview The mechanism occurs in seven steps , involving: 1. The mechanism begins with the protonation of the acetic acid. 2. The π electrons of the carboxyl group, , migrate to pick up the positive charge. 3. The oxygen of the alcohol molecule attacks the carbocation. 4. The oxonium ion that forms loses a proton.

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    Study Guide 5. One of the hydroxyl groups is protonated to form an oxonium ion. 6. An unshared pair of electrons on another hydroxy group reestablishes the carbonyl group, with the loss of a water molecule. 7. The oxonium ion loses a proton, which leads to the production of the ester. 6. Alkyl Sulfonate Formation Alcohols can be converted into alkyl sulfonates , which are sulfonic acid esters.

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    Study Guide Common sulfonyl chlorides • Tosyl chloride (tosylates) • Mesyl chloride (mesylates) • Triflyl chloride (triflates) Importance of sulfonates • They are much better leaving groups than – OH. • Their stability comes from resonance stabilization .

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    Study Guide Alcohols are often converted into sulfonates before S N 1 or S N 2 reactions to improve reaction rate and yield. 7. Oxidation of Alcohols Oxidation reactions of alcohols depend strongly on whether the alcohol is primary or secondary . Formation of Aldehydes and Ketones Primary alcohols • Oxidize to aldehydes using mild oxidizing agents . • Strong oxidizing agents would further oxidize aldehydes to acids.

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    Study Guide Common mild oxidants: • PCC • MnO ₂ • Sarett – Collins reagent Secondary alcohols • Oxidize to ketones . • Ketones resist further oxidation. Following are several examples of the oxidation of primary alcohols:

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