Aldehydes and Ketones Reactions - Online Tutor, Practice Problems & Exam Prep

Reduction reactions involving alcohols are key transformations in chemistry where aldehydes and ketones are reduced to form alcohols. The reducing agent, sodium borohydride (NaBH₄), plays a crucial role in this process by adding hydrogen atoms to the carbon-oxygen and carbon-carbon bonds, effectively increasing the carbon-hydrogen bonds. This results in the loss of a pi bond as the carbon can have a maximum of four bonds, and oxygen prefers to have two. By understanding this mechanism, one can predict the conversion of aldehydes or ketones into alcohols, which is an essential concept in both general and organic chemistry.

Aldehyde and Ketone Reactions

In this video we're going to take a look at alcohol reactions and in particular reduction reactions. So here we're going to say the reducing agent of sodium borohydride, which is NABH₄ dissolved in water, reacts with an alcohol. Now reducing agent. This is just a compound used to reduce aldehydes and ketones. Now the carbonyl oxygen gains an H in this process and the carbonyl carbon also gains in HO. We're creating carbon hydrogen bonds.

Now if we take a look here, we're looking at reduction, so we're moving from left to right. Notice that we're ignoring the carbon dioxide, this carbonic acid and this hydrocarbon when it comes to reduction, at least in Gen. chem. When you get to organic, it's going to expand even more. But for right now, all you need to worry about is when we're talking about reduction, we're talking about aldehydes or ketones being reduced into some type of alcohol.

And like we said, we're adding an H to the carbonyl carbon and an H to the carbonyl oxygen. So if I'm adding an H to this carbon, I have to get rid of one of these π bonds because carbon can't make 5 bonds. Oxygen is ideal, number of bonds is 2, so it's lost a π bond of carbon. So to make up for that it also gains an H.

OK, so that's basically what reduction will be. Locate the carbonyl carbon, add an H to the carbon, add an H to the oxygen, and basically take away their π bond. In that way, you're going to transform an aldehyde or ketone into some type of alcohol.

Aldehyde and Ketone Reactions Example

Here it says determine the alcohol product formed in the following reaction. So here we have a ketone and it's being reduced with sodium boral hydride over water to produce some type of alcohol. Remember fundamentally what's going on in a reduction here?

Well, basically we're adding a hydrogen to the carbonyl carbon, we're adding a hydrogen to the carbonyl oxygen, and we're removing the π bond between the carbon and oxygen. So you're going to have chapter three, Chapter 2. Here goes the carbon. We're adding an H to it.

Here's a carbonyl oxygen, we're adding an H to it and we got rid of the π bond between the carbon and oxygen. And then we write in the rest, chapter 2, Chapter 3. So this would be the alcohol that we create from the reduction of our ketone reactant.

Determine the alcohol product formed in the following reaction.

• A.
• B.
• C.
• D.

Name the alcohol product formed from the following reduction reaction.

• A. 5-ethyl-2-pentanol
• B. 3-methyl-6-heptanol
• C. 5-methyl-2-heptanol
• D. 3-methyl-5-pentanal

Which of the following compounds could not be reduced?

• A. 2,2-dimethylpentane
• B. 2-methyl-1-pentanal
• C. 3-ethyl-2-heptanone
• D. 4-bromoheptanoic acid

Here’s what students ask on this topic:

Which of the following most typically characterizes the reactions of aldehydes and ketones?

Aldehydes and ketones are characterized by their carbonyl group, which is a carbon atom double-bonded to an oxygen atom. This carbonyl group is highly reactive, making aldehydes and ketones susceptible to a variety of chemical reactions. The most typical reactions they undergo include:

1. Nucleophilic Addition: Because the carbonyl carbon is electrophilic (electron-poor), it is susceptible to attack by nucleophiles (electron-rich species). This is the most common type of reaction for aldehydes and ketones, leading to the formation of alcohols after the addition of a nucleophile and a proton.
2. Condensation Reactions: Aldehydes and ketones can react with each other or with other carbonyl-containing compounds in condensation reactions, such as the aldol condensation, to form larger molecules.
3. Oxidation and Reduction: Aldehydes can be easily oxidized to carboxylic acids, while ketones are generally resistant to oxidation. Both aldehydes and ketones can be reduced to alcohols.
4. Formation of Hemiacetals and Acetals: In the presence of alcohols, aldehydes and ketones can form hemiacetals and acetals, which are important in carbohydrate chemistry.

These reactions are central to organic chemistry.

What kind of reactions do aldehydes and ketones undergo?

Aldehydes and ketones are carbonyl compounds that undergo a variety of chemical reactions due to the reactivity of their carbonyl group C=O. Here are some of the common types of reactions they participate in:

1. Nucleophilic Addition Reactions: Because the carbon in the carbonyl group is electrophilic, nucleophiles readily add to it. This is the most characteristic reaction of aldehydes and ketones. Examples include the addition of water to form hydrates, alcohol to form hemiacetals and acetals, hydrogen cyanide to form cyanohydrins, and sodium bisulfite to form bisulfite adducts.
2. Reduction: Aldehydes can be reduced to primary alcohols, and ketones to secondary alcohols, using reducing agents like sodium borohydride NaBH4 or lithium aluminum hydride LiAlH4.
3. Oxidation: Aldehydes are easily oxidized to carboxylic acids with oxidizing agents like potassium permanganate KMnO4 or chromium trioxide CrO3. Ketones are generally resistant to oxidation without breaking the carbon skeleton.
4. Condensation Reactions: These include aldol condensation where two aldehydes or one aldehyde and one ketone react in the presence of a base to form a β-hydroxy aldehyde or ketone, which can further dehydrate to form an α,β-unsaturated carbonyl compound.

Why do aldehydes and ketones undergo nucleophilic addition reactions?

Aldehydes and ketones undergo nucleophilic addition reactions primarily due to the polar nature of their carbonyl group. The carbonyl group consists of a carbon atom double-bonded to an oxygen atom. Oxygen is more electronegative than carbon, which means it pulls the shared electrons in the double bond towards itself, creating a partial negative charge on the oxygen and a partial positive charge on the carbon.

This polarization makes the carbon atom electrophilic, or electron-deficient, rendering it susceptible to attack by nucleophiles. Nucleophiles are species that have a pair of electrons to donate and are attracted to positive or electron-deficient sites.

In the nucleophilic addition reaction, the nucleophile attacks the electrophilic carbon, forming a new bond. This process adds to the carbon atom and breaks the pi bond of the carbonyl group, leading to the formation of a single bond with oxygen, which can then pick up a proton (H⁺) from the environment to form a hydroxyl group (-OH). The overall effect is the conversion of the carbonyl group into a new compound with an additional substituent, which is the basis of the nucleophilic addition mechanism for aldehydes and ketones.