Question

Draw structures for compounds that meet the following descriptions: (a) \(\mathrm{C}_{2} \mathrm{H}_{6} \mathrm{O} ;\) one singlet (b) \(\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{Cl} ;\) one doublet and one septet (c) \(\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{Cl}_{2} \mathrm{O} ;\) two triplets (d) \(\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}\); one singlet, one triplet, and one quartet

Short answer

(a) Dimethyl ether; (b) 2-chloropropane; (c) 1,2-dichloroethoxyethane; (d) Ethyl acetate.

Step-by-step solution

Analyze the Compound Formula for (a)

The formula given is \( \mathrm{C}_2 \mathrm{H}_6 \mathrm{O} \). This corresponds to a simple alcohol or ether. To have a single NMR peak or singlet, the molecule must have chemically equivalent hydrogen atoms. Ethanol (\( \mathrm{CH}_3\mathrm{CH}_2\mathrm{OH} \)) and dimethyl ether (\( \mathrm{CH}_3\mathrm{OCH}_3 \)) are possible structures. Since ethanol will show multiplets, dimethyl ether is the correct structure for a singlet.

Determine the Structure for (b)

The given formula is \( \mathrm{C}_3 \mathrm{H}_7 \mathrm{Cl} \). The presence of a doublet and a septet suggests an isopropyl group \( \mathrm{(CH(CH_3)_2)} \). The only structure that fits this without creating additional signals is 2-chloropropane (\( \mathrm{CH_3CHClCH_3} \)).

Identify the Compound for (c)

The formula is \( \mathrm{C}_4 \mathrm{H}_8 \mathrm{Cl}_2 \mathrm{O} \), requiring two triplets. Consider 1,2-dichloroethoxyethane (\( \mathrm{ClCH_2CH_2OCH_2CH_2Cl} \)). The two sets of \(\mathrm{CH}_2\) groups between chlorine and oxygen create two triplet splits in NMR due to neighboring \(\mathrm{CH}_2\) groups (n+1 rule).

Formulate the Compound for (d)

The molecular formula \( \mathrm{C}_4 \mathrm{H}_8 \mathrm{O}_2 \) suggests potential esters or ethers. A singlet, triplet, and quartet pattern corresponds with ethyl acetate (\( \mathrm{CH_3COOCH_2CH_3} \)). The \( \mathrm{CH}_3 \) from the acetate gives a singlet, the \( \mathrm{CH_2} \) group adjacent to \( \mathrm{O} \) gives a quartet, and the terminal \( \mathrm{CH}_3 \) gives a triplet.

Key concepts

Chemical Structure Determination

Understanding the chemical structure of a compound is vital in predicting its properties and interactions. When given a molecular formula, such as \( \mathrm{C}_2 \mathrm{H}_6 \mathrm{O} \), we need to consider possible structures that fit the description. For formula (a), the structure should have chemically equivalent hydrogen atoms leading to a singlet in NMR.

• Ethanol would show multiplets due to distinct environments around its hydrogens.
• Dimethyl ether, however, maintains chemical equivalency with all hydrogens in an identical environment, leading it to produce a singlet.

Structural determination involves recognizing functional groups and their characteristic signals, helping to predict molecular behavior. In several exercises, the NMR spectrum can aid by revealing which atoms are in similar environments, thereby deducing the compound's full structure.

Chemical Equivalency in NMR

Chemical equivalency in NMR refers to the condition where hydrogen atoms experience the same magnetic environment. In NMR, such atoms produce the same resonance signal, resulting in simplified spectra. This concept is essential for interpreting NMR spectra as seen in the dimethyl ether example, where all hydrogens are symmetrically equivalent.
Consider another compound, 2-chloropropane \( ( \mathrm{CH}_3 \mathrm{CHClCH}_3) \), with a clearer explanation:

• Here, the two methyl groups are equivalent, both producing a doublet due to neighboring hydrogens on the central carbon.
• The central hydrogen is affected equally by both \( \mathrm{CH}_3 \) groups, leading to a septet in the NMR spectrum.

By recognizing equivalent hydrogens, predictions about the number and type of NMR signals a molecule will produce can accurately be made, helping in structure deciphering.

NMR Peak Splitting

NMR peak splitting occurs due to the interaction between non-equivalent neighbor hydrogens. The 'n+1 rule' is a guiding principle where the number of splits in a peak is equal to the number of adjacent hydrogens plus one. This splitting helps identify the proximity and type of neighboring atoms in a molecule.
Take, for example, 1,2-dichloroethoxyethane. It produces two triplet peaks in its NMR spectrum:

• The \( \text{CH}_2 \) groups at either side of the oxygen bridge are adjacent to another \( \text{CH}_2 \), causing each peak to split into a triplet.
• This is due to the mutual coupling, demonstrating the n+1 rule.

Understanding and predicting the splitting patterns can lead to insights into the closeness and arrangement of atoms in a molecule, crucial for accurate chemical structure assignment.

Organic Compound Identification

Identifying organic compounds involves piecing together data from various analytical techniques, especially NMR. NMR provides a magnetic profile of a compound’s nuclei, offering clues about its molecular framework.
Consider ethyl acetate \( (\mathrm{CH}_3\mathrm{COOCH}_2\mathrm{CH}_3) \) as an example:

• The acetate's methyl group gives a singlet because it’s isolated from other hydrogen environments.
• The methylene group connected to oxygen forms a quartet due to the three neighboring hydrogens.
• Lastly, the methylene group at the terminal end of the chain forms a triplet because of the two adjacent hydrogens of the methyl group.

By analyzing NMR signals like these, chemists can decipher the connectivity, length, and branching of carbon chains, making it possible to deduce not just the skeletal structure but also substituent groups and functional conformations of organic compounds.