It is presentation on the topic of ketones and aldehydes reaction, Assignments of Chemistry

Download It is presentation on the topic of ketones and aldehydes reaction and more Assignments Chemistry in PDF only on Docsity! REACTIONS OF ALDEHYDES AND KETONES ADDITIONS OF ALCOHOLS  Alcohols add reversibly to the carbonyl double bond of aldehydes and ketones to give addition products which are generally called Hemiacetals and hemiketals (hemi, Greek, half) .  Aldehydes give more of addition products at equilibrium than do ketones .  To achieve effective hemiacetal or acetal formation, two additional features must be implemented. First, an acid catalyst must be used because alcohol is a weak nucleophile; and second, the water produced with the acetal must be removed from the reaction . Formation of Hemiacetals / Hemiketals : Hemiacetals or hemiketals are a functional group in which –OH group and alkoxy group ( OR ) are attached with same carbon . Explanation : • Aldehydes combine with alcohols in the presence of HCl gas to form acetals . • HCl gas acts as a catalyst . • Both alcohol and gas must be dry. Mechanism : A nucleophilic substitution of an OH group 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: 1. The proton produced by the dissociation of hydrochloric acid protonates the alcohol molecule in an acid‐base reaction. 2. An unshared electron pair from the hydroxyl oxygen of the hemiacetal removes a proton from the protonated alcohol . 3. The oxonium ion is lost from the hemiacetal as a molecule of water. 4. A second molecule of alcohol attacks the carbonyl carbon that is forming the protonated acetal. 5. The oxonium ion loses a proton to an alcohol molecule, liberating the acetal. Note : Ketones do not undergo ketal formation when it is treated with simple alcohols and gaseous HCl ,unless a special technique is used . However ,a cyclic ketal is formed when a ketone is treated with an excess of a 1,2-diol in the presence of a trace of an acid .