The number of compounds which undergo aldol condensation in the following list of compounds are:
methanal, 2-methylpentanal, cyclohexan-1-one, benzaldehyde, l-phenylpropanone, phenylacetaldehyde,2,2-dimethylbutanal, benzophenone, propanone,

Explanation

To determine the number of compounds that undergo aldol condensation from the given list, we need to identify which compounds have alpha-hydrogens. Aldol condensation requires the presence of alpha-hydrogens, as they are necessary for the formation of the enolate ion that participates in the reaction.

### Step-by-Step Solution:

1. **Identify the Compounds:**
The compounds provided are:
- Methanal
- 2-Methylpentanal
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