are often used in conjunction with sterically hindered anhydride donors when synthesizing aldehydes, because the relatively weak reducer will react preferentially with the acid chloride starting material, leaving the aldehyde product unreacted. The reducing agent DIBAL-H (Diisobutylaluminium hydride) is often used for this purpose: though it normally reduces all carbonyls, it can stop reducing at the aldehyde if only one equivalent is used at low temperatures. LiAl(OtBu)3 (formed from LiAlH4 and tBuOH in situ) can also stop reducing at the aldehyde, through a similar mechanism to DIBAL-H. Alternative methods The traditional method of forming aldehydes without reducing to alcohols - by using hindered hydrides and reactive carbonyls - is limited by its narrow substrate scope and great dependence on reaction conditions. One workaround to avoid this method is to reduce the carboxylic acid derivative all the way down to an alcohol, then oxidize the alcohol back to an aldehyde. Other alternatives include forming a thioester or a Weinreb amide, then reducing the new species to an aldehyde through the Fukuyama reduction or Weinreb reaction respectively, or using catalytic hydrogenation as in the Rosenmund reaction. In the Fukuyama reduction, a carboxylic acid is first converted to a thioester through addition of a thiol (with a mechanism similar to esterification). The thioester is then reduced to an aldehyde by a silyl hydride with a palladium catalyst. In the Weinreb ketone synthesis, an acyl chloride is first converted to the Weinreb amide, then treated with an organometallic reagent to form a ketone, or lithium aluminum hydride to form an aldehyde: The Weinreb amide is reduced via a stable chelate, rather than the electrophilic carbonyl that is formed through metal hydride reductions; the chelate is therefore only reduced once, as illustrated below: The Rosenmund reaction reduces acyl chlorides to aldehydes using hydrogen gas with a catalyst of palladium on barium sulfate, whose small surface area prevents over-reduction. Aldehydes and ketones to alkanes Aldehydes and ketones can be reduced not only to alcohols but also to alkanes. Some reactions for this transformation include the Clemmensen reduction (in strongly acidic conditions) and the Wolff–Kishner reduction (in strongly basic conditions), as well as the various modifications of Wolff-Kishner reaction. The Caglioti modification, for instance, uses tosylhydrazone with a hydride donor in milder conditions with no base; the Myers modification substitutes hydrazine with bis(tert-butyldimethylsilyl)-hydrazine, uses milder conditions at room temperature, and is rapid and efficient. Aromatic carbonyls Aromatic