Question 19 of 32intermediate🔧 ApplyShort Answer2 marks

Draw the cis and trans structures of hex-2-ene. Which isomer will have higher b.p. and why?

Correct Answer

cis-hex-2-ene:

CH3C3H7∣∣C=C∣∣HH\begin{matrix} CH_3 & & C_3H_7 \\ | & & | \\ C = C \\ | & & | \\ H & & H \end{matrix}

(Same side)

trans-hex-2-ene:

CH3H∣∣C=C∣∣HC3H7\begin{matrix} CH_3 & & H \\ | & & | \\ C = C \\ | & & | \\ H & & C_3H_7 \end{matrix}

(Opposite sides)

The cis isomer will have a higher boiling point. This is because the cis form of an alkene is more polar than the trans form. In the trans isomer, the dipole moments of the bonds cancel out, making it almost non-polar. Due to its higher polarity, the cis isomer experiences stronger intermolecular dipole-dipole interactions, requiring more heat to break, thus resulting in a higher boiling point.

Exercise: EXERCISES | Q: 9.9 | (Page Page 33)
For More Understanding

Explanation

The question asks for the cis and trans structures of hex-2-ene and a comparison of their boiling points. Based on the provided context, the cis form of alkenes is more polar than the trans form because the dipole moments do not cancel out as they do in the trans form. This higher polarity in the cis isomer leads to stronger intermolecular forces, giving it a higher boiling point compared to the trans isomer.

Solution Steps

  1. Step 1: Draw the structures of cis and trans hex-2-ene by placing the CH3CH_3 and C3H7C_3H_7 groups on the same side for cis, and opposite sides for trans.

  2. Step 2: Identify which isomer has a higher boiling point. The cis isomer is more polar than the trans isomer.

  3. Step 3: Explain the reason. In the trans form, dipole moments cancel out making it non-polar, whereas the cis form has a net dipole moment, leading to stronger intermolecular forces and a higher boiling point.

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