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Organic Chemistry II - Aldehydes and Ketones

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    Study Guide Organic Chemistry II – Aldehydes and Ketones 1. Ketones What Are Ketones? Ketones are organic compounds where an oxygen atom is bonded to a carbon atom , which in turn is bonded to two other carbon atoms . This structure is called a carbonyl group (C=O), and it is a key 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 Ketones Ketones can be named using two systems: the common system and the IUPAC system .

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    Study Guide 1. Common System : o In the common system, ketones are named by identifying the groups attached to the carbonyl carbon and then adding the word "ketone" . o For example: • Ethyl Methyl Ketone • Phenyl Propyl Ketone • Phenyl p - Tolyl Ketone 2. IUPAC System : o In the IUPAC system, ketones are named by taking the alkane name, dropping the - e ending, and replacing it with - one .

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    Study Guide For example: • 2,2 - dimethyl - 3 - hexanone • 3 - methyl - 2 - butanone 2. Special Cases for Aromatic Ketones • In the IUPAC system , aromatic ketones are considered as benzene - substituted aliphatic ketones . For example: o 1 - phenylethanone (commonly known as acetophenone ): o Diphenylmethanone (commonly known as benzophenone ): Many aromatic ketones keep their common names even in the IUPAC system .

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    Study Guide Key Takeaway • Ketones have a carbonyl group (C=O) , where the carbon is bonded to two other carbon atoms. • Ketones are named using either the common system or the IUPAC system , depending on the type of compound. • For aromatic ketones , the IUPAC system treats them as benzene - substituted aliphatic ketones , but many keep their common names (like acetophenone and benzophenone ). 2. Synthesis of Ketones Ketones can be synthesized by several methods, much like aldehydes. Here are the most common ways to prepare ketones: 1. Oxidation of Secondary Alcohols • The oxidation of secondary alcohols is a common method to prepare ketones. In this case, the hydroxyl group (OH) of the alcohol is oxidized into a carbonyl group (C=O) , turning the alcohol into a ketone. • Strong oxidizing agents, such as potassium dichromate (K ₂ Cr ₂ O ₇ ) and chromic acid (H ₂ CrO ₄ ) , are used for this oxidation. Example: o 2 - propanol (secondary alcohol) is oxidized to 2 - propanone (acetone) .

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    Study Guide 2. Hydration of Alkynes • When water is added to an alkyne (a compound with a triple bond), it forms an unstable vinyl alcohol . This then undergoes tautomerization (a rearrangement) to produce a ketone. Example: o The hydration of propene (CH ₂ =CH - CH ₃ ) gives 2 - propanone (acetone). 3. Ozonolysis of Alkenes • Ozonolysis is a reaction where ozone (O ₃ ) cleaves a double bond of an alkene. When alkenes (like 1,2 - dimethylpropene ) undergo ozonolysis, two ketones can form from the cleaved double bond. Example: o The ozonolysis of 1,2 - dimethylpropene forms 2 - propanone (acetone) and ethanal (acetaldehyde) .

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    Study Guide 4. Friedel - Crafts Acylation • The Friedel - Crafts acylation is used to prepare aromatic ketones . In this reaction, benzene reacts with an acyl chloride (like acetyl chloride) in the presence of AlCl ₃ (a catalyst), resulting in the formation of acetophenone (a ketone with an aromatic group attached). Example: o The reaction of benzene with acetyl chloride forms acetophenone . 5. Lithium Dialkylcuprates (Gilman Reagents) • A lithium dialkylcuprate (also called a Gilman reagent ) reacts with an acyl chloride at low temperatures to form a ketone. Example: o Acetophenone can be prepared by reacting a Gilman reagent with an acyl chloride . 6. Grignard Reagents • Grignard reagents (such as methyl magnesium bromide ) can also be used to form ketones. In this method, a Grignard reagent reacts with a nitrile to form a ketone .

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    Study Guide Example: • Acetone is produced by reacting methyl magnesium bromide (CH ₃ MgBr) with methyl nitrile (CH ₃ CN) . Key Takeaway 1. 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 Ketones 1. Reactions of Aldehydes and Ketones Aldehydes and ketones undergo a variety of reactions that lead to many different products. The most common reactions are nucleophilic addition reactions, which lead to the formation of alcohols, alkenes, diols, cyanohydrins (RCH(OH)C&tbond;N), and imines R 2 C&dbond;NR), to mention a few representative examples. 2. Reactions of carbonyl groups The main reactions of the carbonyl group are nucleophilic additions to the carbon ‐ oxygen double bond. As shown below, this addition consists of adding a nucleophile and a hydrogen across the carbon ‐ oxygen double bond.

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    Study Guide Due to differences in electronegativities, the carbonyl group is polarized. The carbon atom has a partial positive charge, and the oxygen atom has a partially negative charge. Aldehydes are usually more reactive toward nucleophilic substitutions than ketones because of both steric and electronic effects. In aldehydes, the relatively small hydrogen atom is attached to one side of the carbonyl group, while a larger R group is affi xed to the other side. In ketones, however, R groups are attached to both sides of the carbonyl group. Thus, steric hindrance is less in aldehydes than in ketones. Electronically, aldehydes have only one R group to supply electrons toward the partially positive carbonyl 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 positive charge on this atom and the weaker it will become as a nucleus. 3. Addition of water The addition of water to an aldehyde results in the formation of a hydrate.

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    Study Guide The 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 ‐ base reaction. Small amounts of acids and bases catalyze this reaction. This occurs because the addition of acid causes a protonation of the oxygen of the carbonyl group, leading to the formation of a full positive charge on the carbonyl carbon, making the carbon a good nucleus. Adding hydroxyl ions changes the nucleophile from water (a weak nucleophile) to a hydroxide ion (a strong nucleophile). Ketones usually do not form stable hydrates. 4 . Addition of alcohol Reactions of aldehydes with alcohols produce either hemiacetals (a functional group consisting of one — OH group and one — OR group bonded to the same carbon) or acetals (a functional group consisting of two — OR groups bonded to the same carbon), depending upon conditions. Mixing the two reactants together produces the hemiacetal. Mixing the two reactants with hydrochloric acid produces an acetal. For example, the reactio n of methanol with ethanal produces the following results:

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    Study Guide A nucleophilic substitution of an OH group for the double bond of the carbonyl group forms the hemiacetal 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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