Question

Identify the wrong statement about alkenes. (1) The boiling points, melting points and specific gravities show a gradual increase with the increase in molecular mass (2) These are less volatile and have more melting points and boiling points than corresponding alkanes (3) Cis isomers have lower boiling points than trans isomers (4) Cis isomers have lower melting points than trans isomers

Short answer

Statement 2 is incorrect.

Step-by-step solution

- Analyze statement 1

Statement 1 says that the boiling points, melting points, and specific gravities show a gradual increase with the increase in molecular mass. This is generally true for alkenes, as larger molecules have stronger van der Waals forces, leading to higher melting points, boiling points, and densities.

- Analyze statement 2

Statement 2 claims that alkenes are less volatile and have higher melting and boiling points than corresponding alkanes. This is incorrect because alkanes, being saturated hydrocarbons, generally have higher melting and boiling points compared to their unsaturated alkene counterparts of similar molecular mass.

- Analyze statement 3

Statement 3 states that cis isomers have lower boiling points than trans isomers. This is true because cis isomers have higher polarity, leading to stronger intermolecular forces compared to trans isomers, which generally result in higher boiling points for trans isomers.

- Analyze statement 4

Statement 4 mentions that cis isomers have lower melting points than trans isomers. This is correct because the molecular packing in cis isomers is less efficient compared to trans isomers, which leads to lower melting points for cis isomers.

- Identify the incorrect statement

After analyzing all statements, identify the incorrect one. Statement 2 is incorrect as alkanes, being more saturated, typically have higher melting and boiling points than alkenes with the same carbon chain length.

Key concepts

boiling point trends

The boiling points of alkenes generally increase as the molecular mass increases. This is due to the stronger van der Waals forces between larger molecules. Van der Waals forces are temporary, random attractions between molecules. As the molecular size and surface area increase, these forces become stronger and require more heat energy to break, resulting in higher boiling points.
To summarize, larger alkenes with more carbon atoms will generally have higher boiling points compared to smaller alkenes. This trend is similar to what we observe in alkanes and other hydrocarbons.

cis and trans isomers

Isomers are molecules with the same molecular formula but different structural arrangements. In alkenes, isomerism often occurs in the form of cis and trans isomers, which differ by the orientation of the substituents around the double bond.

• Cis Isomers: The substituents are on the same side of the double bond.
• Trans Isomers: The substituents are on opposite sides of the double bond.

Cis and trans isomers have different physical properties. For example, cis isomers have higher polarity because the substituents are closer, leading to stronger intermolecular forces. This results in a higher boiling point for cis isomers compared to their trans counterparts. However, cis isomers have less efficient packing in the solid state, leading to lower melting points compared to trans isomers.

volatility of alkenes

Volatility refers to how easily a substance can vaporize. It is closely related to the boiling point of a substance. Lower boiling points indicate higher volatility and vice versa.
Compared to alkanes of similar molecular mass, alkenes are generally more volatile. This is because alkenes have weaker van der Waals forces due to the presence of double bonds. The double bonds introduce kinks in the molecular structure, making the molecules less tightly packed compared to alkanes. As a result, less energy is required to separate the molecules, leading to lower boiling points and higher volatility.

melting point trends

Melting points follow a similar trend to boiling points when it comes to molecular mass. As the size of the alkene increases, so does its melting point. This is again due to the increasing strength of van der Waals forces.
However, the type of isomer also plays a crucial role in determining the melting point. Trans isomers generally have higher melting points compared to cis isomers. This is because trans isomers can pack more efficiently in the solid state, leading to a more stable solid structure that requires more energy to break apart. In contrast, the less efficient packing of cis isomers leads to a less stable structure, resulting in lower melting points.