The iodoform test is positive for compounds containing a methyl ketone group ($-COCH_3$) or a secondary alcohol with a methyl group on the \alpha carbon ($-CH(OH)CH_3$).

1. Ethanal ($CH_3CHO$) contains the $CH_3-C=O$ group. It gives a positive test.
2. Ethanol ($CH_3CH_2OH$) contains the $CH_3-CH(OH)-$ group. It oxidizes to ethanal and gives a positive test.
3. 2-Pentanone ($CH_3-CO-CH_2-CH_2-CH_3$) contains the methyl ketone group ($CH_3-C=O$). It gives a positive test.
4. 3-Pentanone ($CH_3-CH_2-CO-CH_2-CH_3$) contains ethyl groups attached to the carbonyl carbon, not a methyl group. Therefore, it does not undergo the haloform reaction to produce iodoform.

Thus, 3-Pentanone is the compound that does not give an iodoform test.