Q14P - Organic Chemistry (Mcmurry)

Question

Propose structures for compounds that fit the following mass-spectraldata:

(a) A hydrocarbon with M⁺ = 132

(b) A hydrocarbon with M⁺ = 166

(c) A hydrocarbon with M⁺ =  84

Step-by-Step Solution

Answer

Possible structures of molecule:

(a) 

(b)

(c)

Molecular mass

Molecular mass of a substance is the sum of atomic masses of all its atoms in a molecule.

Calculation of molecular formula

• Thesemoleculeshave only hydrogen and carbon. 
• Molecular mass of carbon is 12 and molecular mass of hydrogen is 1.
• Sum of molecular mass of C and H is 12 + 1 = 13
• Now divide the given molecular mass by 13.
• The quotient indicates the number of carbon and number of hydrogen is equal to the quotient plus the remainder. 

Molecular formula of given molecules

(a) A hydrocarbon with molecular weight 132:

• Given by:$\frac{132}{13}=10\frac{2}{13}$
• This indicates that there are 10 carbon and 12hydrogens.

• So, the formula is C₁₀H₁₂              

(b) A hydrocarbon with molecular weight 166:

• Given by: $\frac{166}{13}=12\frac{10}{13}$^{$\frac{84}{13}$}
• This indicates that there are 12 carbon and 22 hydrogen.
• So, the formula is  C₁₂H₂₂