Problem 78 - Chemistry: The Central Science in SI Units, Global Edition

Question

A new compound is made that has a \(\mathrm{C}-\mathrm{N}\) bond length of \(118 \mathrm{pm}\). Is this bond likely to be a single, double, or triple C-N bond?

Step-by-Step Solution

Answer

The given C-N bond length is \(118\mathrm{~pm}\), which is closest to the standard triple C-N bond length (\(120\mathrm{~pm}\)). Therefore, it is likely that the C-N bond in the compound is a triple bond.

Step 1: Gather standard bond lengths for C-N bonds

First, we will need to know the standard bond lengths for single, double, and triple C-N bonds. These values can be found in a chemistry reference book or online. The standard bond lengths are as follows: - Single C-N bond: \(147\mathrm{~pm}\) - Double C-N bond: \(130\mathrm{~pm}\) - Triple C-N bond: \(120\mathrm{~pm}\).

Step 2: Compare the given bond length to standard bond lengths

Now that we have the standard bond lengths of single, double, and triple C-N bonds, we can compare the given bond length (\(118\mathrm{~pm}\)) to these values: - \(147\mathrm{~pm} - 118\mathrm{~pm} = 29\mathrm{~pm}\) (Difference between single bond and given bond length) - \(130\mathrm{~pm} - 118\mathrm{~pm} = 12\mathrm{~pm}\) (Difference between double bond and given bond length) - \(120\mathrm{~pm} - 118\mathrm{~pm} = 2\mathrm{~pm}\) (Difference between triple bond and given bond length)

Step 3: Determine the likely bond type based on bond length differences

Looking at the differences calculated in Step 2, the smallest difference is between the given bond length and the triple bond length, with just a difference of 2 pm. Therefore, it is likely that the C-N bond in the compound is a triple bond.

Key Concepts

Single C-N Bond

A single C-N bond is a common type of bond found in many organic compounds. It is formed when one pair of electrons is shared between a carbon and a nitrogen atom. This sharing of electrons allows both atoms to achieve a more stable configuration.

The length of a typical single C-N bond is quite significant at approximately 147 pm. This measurement arises because the bond involves just one pair of electrons, resulting in a relatively lower degree of overlap between the atomic orbitals of carbon and nitrogen.

Such bonds are generally flexible and allow rotational movement around the bond axis, making them quite useful in various organic molecules where movement is needed. Single C-N bonds are often found in compounds like amines and amides, which are important building blocks in both biological and synthetic chemistry.

• Bond length: 147 pm
• Type: Single bond
• Common in: Amines, amides

Double C-N Bond

A double C-N bond involves the sharing of two pairs of electrons between carbon and nitrogen atoms. This type of bond is stronger and shorter than a single C-N bond. The presence of two electron pairs increases the electron density between the atoms, enhancing the strength of the bond.

The double C-N bond typically measures about 130 pm in length. This reduction in length compared to a single C-N bond is due to the additional electron pair, which brings the atoms closer together.

Double C-N bonds are commonly found in compounds such as imines and enamines. These structures play a key role in various chemical reactions, serving as intermediates in many synthetic processes. This added bond strength makes the C-N linkage more rigid, limiting its rotation compared to a single bond.

• Bond length: 130 pm
• Type: Double bond
• Common in: Imines, enamines

Triple C-N Bond

The triple C-N bond is the strongest and shortest amongst the types of carbon-nitrogen bonds. It is characterized by the sharing of three pairs of electrons between the carbon and nitrogen atoms, leading to a very stable and tightly bound structure.

Due to the high electron density between the atoms, the stable nature of the bond results in a bond length of approximately 120 pm. This is considerably shorter than both single and double C-N bonds.

Triple C-N bonds are typically found within molecules like nitriles and cyanides. Such bonds are extremely rigid due to the extra electron pairs, and they don't easily allow for rotation. This rigidity makes compounds with triple C-N bonds very linear in their geometry. Understanding this bond is crucial for its role in many industrial and chemical applications.

• Bond length: 120 pm
• Type: Triple bond
• Common in: Nitriles, cyanides