Are all alkenes chiral?

No, not all alkenes are chiral.

While alkenes themselves don't inherently possess a chiral center in the traditional sense (a carbon atom bonded to four different groups), chirality can arise in alkenes under specific circumstances. The presence of a carbon-carbon double bond (C=C) gives rise to cis and trans isomers (also known as E and Z isomers), but these are not necessarily chiral.

Here's a breakdown:

Achiral Alkenes: Most simple alkenes like ethene (CH₂=CH₂) or propene (CH₃CH=CH₂) are achiral because they possess a plane of symmetry or a center of inversion. Even substituted alkenes are achiral if the substituents are arranged symmetrically. For example, cis-1,2-dichloroethene is achiral due to a plane of symmetry.
Chiral Alkenes (Alkenes with Chiral Centers): An alkene molecule becomes chiral if it contains a stereogenic center elsewhere in the molecule apart from the carbon-carbon double bond. The double bond itself doesn't become a chiral center, but the presence of an existing chiral center within the same molecule causes the entire molecule to be chiral.
Axial Chirality (Atropisomerism): In some cases, chirality can arise due to restricted rotation around a bond, even if there are no classical chiral centers. This is called axial chirality or atropisomerism. While this is typically observed in biaryl compounds, it's less common for simple alkenes. This kind of chirality requires bulky substituents that prevent free rotation about a single bond connected to the alkene.

Examples:

Achiral Alkene: But-2-ene (CH₃CH=CHCH₃) exists as cis and trans isomers. Neither of these is chiral; the cis isomer has a plane of symmetry, and the trans isomer also possesses a center of symmetry.
Chiral Alkene (with a Chiral Center Elsewhere): Consider an alkene molecule with a chiral carbon elsewhere in the molecule, like 3-methylhex-2-ene with a chiral substituent at position 3. The presence of that chiral center makes the entire molecule chiral.

In Summary:

Alkenes per se are not inherently chiral, but chirality can be introduced if:

A stereogenic center exists elsewhere in the molecule.
Restricted rotation around a single bond adjacent to the alkene leads to axial chirality, though this is less common with simple alkenes.

Therefore, it's important to consider the entire molecular structure when determining if an alkene-containing compound is chiral.