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Organic Chemistry II - Aldehydes and Ketones

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Study GuideOrganic Chemistry II–Aldehydes and Ketones1.KetonesWhat Are Ketones?Ketonesare organic compounds where anoxygen atomis bonded to acarbon atom, which in turnis bonded totwo other carbon atoms. This structure is called acarbonyl group(C=O), and it is akey feature of ketones.Examples of Ketones:1.2-butanone (Methyl Ethyl Ketone)2.1-phenylethanone (Methyl Phenyl Ketone)3.Diphenyl Methanone (Diphenyl Ketone)1.Naming KetonesKetones can be named using two systems: thecommon systemand theIUPAC system.

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Study Guide1.Common System:oIn the common system, ketones are named by identifying the groups attached to thecarbonyl carbonand then adding the word"ketone".oFor example:•Ethyl Methyl Ketone•Phenyl Propyl Ketone•Phenyl p-Tolyl Ketone2.IUPAC System:oIn the IUPAC system, ketones are named by taking the alkane name, dropping the-eending, and replacing it with-one.

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Study GuideFor example:•2,2-dimethyl-3-hexanone•3-methyl-2-butanone2.Special Cases for Aromatic Ketones•In theIUPAC system, aromatic ketones are considered asbenzene-substituted aliphaticketones. For example:o1-phenylethanone(commonly known asacetophenone):oDiphenylmethanone(commonly known asbenzophenone):Many aromatic ketones keep theircommon nameseven in theIUPAC system.

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Study GuideKey Takeaway•Ketoneshave acarbonyl group (C=O), where the carbon is bonded to two other carbonatoms.•Ketones are named using either thecommon systemor theIUPAC system, depending onthe type of compound.•Foraromatic ketones, the IUPAC system treats them asbenzene-substituted aliphaticketones, but many keep theircommon names(likeacetophenoneandbenzophenone).2.Synthesis of KetonesKetones can be synthesized by several methods, much like aldehydes. Here are the most commonways to prepare ketones:1. Oxidation of Secondary Alcohols•The oxidation ofsecondary alcoholsis a common method to prepare ketones. In this case,thehydroxyl group (OH)of the alcohol is oxidized into acarbonyl group (C=O), turning thealcohol into a ketone.•Strong oxidizing agents, such aspotassium dichromate (K₂Cr₂O₇)andchromic acid(H₂CrO₄), are used for this oxidation.Example:o2-propanol (secondary alcohol)is oxidized to2-propanone (acetone).

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Study Guide2. Hydration of Alkynes•When water is added to analkyne(a compound with a triple bond), it forms anunstablevinyl alcohol. This then undergoestautomerization(a rearrangement) to produce a ketone.Example:oThehydration of propene (CH₂=CH-CH₃)gives2-propanone(acetone).3. Ozonolysis of Alkenes•Ozonolysis is a reaction where ozone (O₃) cleaves a double bond of an alkene. Whenalkenes(like1,2-dimethylpropene) undergo ozonolysis,two ketonescan form from thecleaved double bond.Example:oThe ozonolysis of1,2-dimethylpropeneforms2-propanone (acetone)andethanal(acetaldehyde).

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Study Guide4. Friedel-Crafts Acylation•TheFriedel-Crafts acylationis used to preparearomatic ketones. In this reaction,benzenereacts with anacyl chloride(like acetyl chloride) in the presence ofAlCl₃(a catalyst),resulting in the formation ofacetophenone(a ketone with an aromatic group attached).Example:oThe reaction ofbenzenewithacetyl chlorideformsacetophenone.5. Lithium Dialkylcuprates (Gilman Reagents)•Alithium dialkylcuprate(also called aGilman reagent) reacts with anacyl chlorideatlowtemperaturesto form a ketone.Example:oAcetophenonecan be prepared by reacting aGilman reagentwith anacylchloride.6. Grignard Reagents•Grignard reagents (such asmethyl magnesium bromide) can also be used to form ketones.In this method, aGrignard reagentreacts with anitrileto form aketone.

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Study GuideExample:•Acetoneis produced by reactingmethyl magnesium bromide (CH₃MgBr)withmethyl nitrile (CH₃CN).Key Takeaway1.Oxidation of secondary alcohols: Converts alcohols to ketones.2.Hydration of alkynes: Adds water to alkynes to form ketones.3.Ozonolysis of alkenes: Breaks double bonds in alkenes to form ketones.4.Friedel-Crafts acylation: Prepares aromatic ketones using acyl chlorides.5.Lithium dialkylcuprates: A method for making ketones from acyl chlorides.6.Grignard reagents: Reacting Grignard reagents with nitriles to form ketones.3.Reactions of Aldehydes and Ketones1.Reactions of Aldehydes and KetonesAldehydes and ketones undergo a variety of reactions that lead to many different products. The mostcommon reactions are nucleophilic addition reactions, which lead to the formation of alcohols,alkenes, diols, cyanohydrins (RCH(OH)C&tbond;N), and imines R2C&dbond;NR), to mention a fewrepresentative examples.2.Reactions of carbonyl groupsThe main reactions of the carbonyl group are nucleophilic additions to the carbon‐oxygen doublebond. As shown below, this addition consists of adding a nucleophile and a hydrogen across thecarbon‐oxygen double bond.

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Study GuideDue to differences in electronegativities, the carbonyl group is polarized. The carbon atom has apartial positive charge, and the oxygen atom has a partially negative charge.Aldehydes are usually more reactive toward nucleophilic substitutions than ketones because of bothsteric and electronic effects. In aldehydes, the relatively small hydrogen atom is attached to one sideof the carbonyl group, while a larger R group is affixed to the other side. In ketones, however, Rgroups are attached to both sides of the carbonyl group. Thus, steric hindrance is less in aldehydesthan in ketones.Electronically, aldehydes have only one R group to supply electrons toward the partially positivecarbonyl carbon, while ketones have two electron‐supplying groups attached to the carbonyl carbon.The greater amount of electrons being supplied to the carbonyl carbon, the less the partial positivecharge on this atom and the weaker it will become as a nucleus.3.Addition of waterThe addition of water to an aldehyde results in the formation of a hydrate.

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Study GuideThe formation of a hydrate proceeds via a nucleophilic addition mechanism.1. Water, acting as a nucleophile, is attracted to the partially positive carbon of the carbonyl group,generating an oxonium ion.2. The oxonium ion liberates a hydrogen ion that is picked up by the oxygen anion in an acid‐basereaction.Small amounts of acids and bases catalyze this reaction. This occurs because the addition of acidcauses a protonation of the oxygen of the carbonyl group, leading to the formation of a full positivecharge on the carbonyl carbon, making the carbon a goodnucleus. Adding hydroxyl ions changes thenucleophile from water (a weak nucleophile) to a hydroxide ion (a strong nucleophile). Ketonesusually do not form stable hydrates.4.Addition of alcoholReactions of aldehydes with alcohols produce eitherhemiacetals(a functional group consisting ofone—OH group and one—OR group bonded to the same carbon) oracetals(a functional groupconsisting of two—OR groups bonded to the same carbon), depending upon conditions. Mixing thetwo reactants together produces the hemiacetal. Mixing the two reactants with hydrochloric acidproduces an acetal. For example, the reaction of methanol with ethanal produces the followingresults:

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Study GuideA nucleophilic substitution of an OH group for the double bond of the carbonyl group forms thehemiacetal through the following mechanism:1. An unshared electron pair on the alcohol's oxygen atom attacks the carbonyl group.2. The loss of a hydrogen ion to the oxygen anion stabilizes the oxonium ion formed in Step 1.The addition of acid to the hemiacetal creates an acetal through the following mechanism:
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Document Details

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

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