Well-known acyl compounds are the acyl chlorides, such as acetyl chloride (CHCOCl) and benzoyl chloride (CHCOCl). These compounds, which are treated as sources of acylium cations, are good reagents for attaching acyl groups to various substrates. Amides (RC(O)NR′) and esters (RC(O)OR′) are classes of acyl compounds, as are ketones (RC(O)R′) and aldehydes (RC(O)H), where R and R′ stand for organyl (or hydrogen in the case of formyl).