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Organic Chemistry II - Alcohols and Ethers

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Study GuideOrganic Chemistry II–Alcohols and Ethers1.Reactions of AlcoholsAlcohols are very important organic compounds because they can be converted into many otheruseful substances. This wide range of reactions is due to the presence of thehydroxyl (–OH) group.In this chapter, we will study how alcohols react and what products are formed under differentconditions.What Can Alcohols Be Converted Into?Alcohols can be converted into:•Metal salts (alkoxides)•Alkyl halides•Esters•Aldehydes•Ketones•Carboxylic acidsEach type of reaction is discussed below in a clear and step-by-step manner.1. Metal Salt (Alkoxide) FormationAcidic nature of alcoholsAlcohols behave asweak acids. They are only slightly weaker acids than water.•(Ka) of alcohols ≈1 × 10⁻¹⁶•(Ka) of water ≈1 × 10⁻¹⁵

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Study GuideReaction with sodium metalWhen ethanol reacts with sodium metal, it formssodium ethoxideand releaseshydrogen gas.This reaction is very similar to the reaction between sodium metal and water, but the reaction withwater occurs faster because water is slightly more acidic.Similar reactions also occur withpotassium metal.Effect of structure on acidity•Acidity of alcoholsdecreasesin the order:Primary > Secondary > TertiaryThis 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•Basicityincreasesfromprimary to tertiary alkoxides.•A weaker acid always has astronger conjugate base.2. Formation of Alkyl Halides from AlcoholsAlcohols can be converted into alkyl halides by reacting withhydrogen halides (HCl, HBr, HI).

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Study GuideReaction mechanism•Primary alcoholsmainly undergoSN2reactions.•Tertiary alcoholsmainly undergoSN1reactions.Heat is usually required to carry out these reactions.Key point:The structure of the alcohol decides whether the reaction followsSN1orSN2.3. Alkyl Chloride Formation Using Thionyl Chloride (SOCl₂)A more efficient method for preparing alkyl chlorides from alcohols usesthionyl chloride (SOCl₂).Advantages of this method•Reaction is fast•Fewer side products•By-products (SO₂and HCl) are gases and escape easilyMechanism (simplified)1.The alcohol first reacts with SOCl₂to form aninorganic ester.2.Chloride ion attacks the carbon atom by anSN2mechanism.3.Products formed arealkyl chloride, SO₂, and HCl.

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Study GuideStereochemical resultSince the reaction mainly follows anSN2pathway, an optically active alcohol gives a product withinverted configuration.4. Formation of Alkyl Bromides and Alkyl IodidesAlkyl bromides•Thionyl bromide is unstable.•Therefore,phosphorus tribromide (PBr₃)is used.Mechanism:1.Alcohol reacts with PBr₃to form a phosphorus ester.

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Study Guide2.Bromide ion replaces the ester group viaSN1orSN2substitution.Alkyl iodides•Prepared in a similar way usingphosphorus triiodide (PI₃).5. Ester Formation (Fischer Esterification)What are esters?Esters are formed whenalcohols react with carboxylic acids.•Ester functional group:–COORFischer esterification process•Alcohol + carboxylic acid•Acid catalyst (H⁺)•Heat is appliedThis reaction isreversible.Improving yield

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Study GuideBecause the reaction is an equilibrium reaction, a higher yield of ester is obtained byremoving wateror ester as it forms.Mechanism overviewThe mechanism occurs inseven steps, involving:1. The mechanism begins with the protonation of the acetic acid.2. Theπelectrons 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 Guide5. 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 theloss of a water molecule.7. The oxonium ion loses a proton, which leads to the production of the ester.6. Alkyl Sulfonate FormationAlcohols can be converted intoalkyl sulfonates, which are sulfonic acid esters.

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Study GuideCommon sulfonyl chlorides•Tosyl chloride (tosylates)•Mesyl chloride (mesylates)•Triflyl chloride (triflates)Importance of sulfonates•They aremuch better leaving groupsthan–OH.•Their stability comes fromresonance stabilization.

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Study GuideAlcohols are often converted into sulfonates beforeSN1orSN2reactions to improve reaction rate andyield.7. Oxidation of AlcoholsOxidation reactions of alcohols depend strongly on whether the alcohol isprimary or secondary.Formation of Aldehydes and KetonesPrimary alcohols•Oxidize toaldehydesusingmild oxidizing agents.•Strong oxidizing agents would further oxidize aldehydes to acids.

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Study GuideCommon mild oxidants:•PCC•MnO₂•Sarett–Collins reagentSecondary alcohols•Oxidize toketones.•Ketones resist further oxidation.Following are several examples of the oxidation of primary alcohols:
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Document Details

University
Texas A&M University
Course
Organic Chemistry II
Subject
Chemistry

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