Question

Which of the following compounds would be most reactive toward a nucleophile? A. Pentanal B. 3-pentanone C. Pentane D. 2-nonanone

Short answer

Pentanal (A) would be the most reactive toward a nucleophile.

Step-by-step solution

- Understand the Nucleophilic Reaction

Nucleophiles are attracted to electron-deficient sites where they can donate a pair of electrons. In organic compounds, such sites are often present at carbonyl carbons (C=O) because the carbon is partially positive.

- Identify the Carbonyl Groups

Examine the given compounds to identify which ones contain carbonyl groups (C=O). Pentanal (A) and 3-pentanone (B) both have carbonyl groups, while pentane (C) is an alkane and 2-nonanone (D) is a ketone.

- Compare Aldehydes and Ketones

Nucleophiles are generally more reactive with aldehydes than ketones because aldehydes have less steric hindrance and a more electron-deficient carbonyl carbon. In this case, pentanal (A) is an aldehyde while 3-pentanone (B) and 2-nonanone (D) are ketones.

- Determine Reactivity of Alkanes

Pentane (C) does not have a carbonyl group or any significantly electron-deficient site, making it the least reactive toward nucleophiles.

- Conclusion

Between the remaining options, pentanal (A), which is an aldehyde, will be more reactive toward nucleophiles compared to the ketones. Therefore, pentanal (A) is the most reactive compound toward a nucleophile.

Key concepts

Nucleophiles

Nucleophiles are species that donate an electron pair to an electron-deficient atom, usually in the context of a chemical reaction. They are essentially 'nucleus-loving' because they seek a positively charged or electron-deficient site in another molecule. Common nucleophiles include

• Hydroxide ion (OH⁻)
• Ammonia (NH₃)
• Alkoxide ion (RO⁻)
• Halide ions (F⁻, Cl⁻, Br⁻, I⁻)

Nucleophiles can attack various sites in a molecule, but they are especially reactive towards carbonyl carbons due to the partial positive charge on the carbon in the carbon-oxygen double bond. This results from the oxygen atom's higher electronegativity, which pulls electron density away from the carbon atom, making it an attractive site for nucleophiles to donate their electron pair.

Carbonyl Groups

Carbonyl groups are functional groups characterized by a carbon atom double-bonded to an oxygen atom (C=O). These groups are highly significant in organic chemistry and are found in several types of compounds, including aldehydes, ketones, carboxylic acids, and esters.

The carbon-oxygen double bond in carbonyl groups is polar because oxygen is more electronegative than carbon. This polarity makes the carbonyl carbon an electron-deficient site, or electrophilic, which attracts nucleophiles.

Understanding where carbonyl groups are located in a molecule helps identify potential sites for nucleophilic attack. For instance:

• Aldehydes: Carbonyl group at the end of the carbon chain
• Ketones: Carbonyl group within the carbon chain

This placement impacts the reactivity of the carbonyl compound towards nucleophiles.

Reactivity of Aldehydes and Ketones

Aldehydes and ketones both contain carbonyl groups, yet they exhibit different reactivities towards nucleophiles. This difference primarily stems from the steric and electronic environments around the carbonyl carbon.

Aldehydes have one hydrogen atom and one organic group attached to the carbonyl carbon, making the carbon more exposed and electron-deficient. This set-up makes aldehydes more reactive towards nucleophiles compared to ketones, which have two organic groups attached to the carbonyl carbon, providing more steric hindrance and electron donation to the carbonyl carbon.

In the case of the exercise provided, pentanal is an aldehyde and thus more reactive towards nucleophiles than the ketones 3-pentanone and 2-nonanone. Additionally, pentane lacks a carbonyl group and isn't significantly electron-deficient, ranking it the least reactive. Aldehydes are generally more reactive than ketones in nucleophilic addition reactions because:

• Less steric hindrance
• More electron deficiency at the carbonyl carbon