See "Acetaldehyde + Sulfur" //www.curezone.org/forums/fm.asp?i=1945039
See "Acetaldehyde + Disulfide Bonds" //www.curezone.org/forums/fm.asp?i=1958887
But when it can't find a sulfur center to glom onto even other carbonyls will do. The simplest illustration of this type of reaction is to consider the potential interaction of two acetaldehyde molecules with each other.
Hydrogens adjacent to a carbonyl group are weakly acidic so that the loss of a proton to a hydroxide ion, forming a water molecule, can result in a resonance-stabilized enolate ion:
The nucleophilic properties of the enolate ion and the polar properties of acetaldehyde favor the formation of an aldol, a molecule with both an aldehyde and alcohol group. In the case of two acetaldehyde molecules, the result is 3-hydroxybutanal:
The exothermic dehydration of this aldol can result in the formation of an alpha-beta unsaturated aldehyde, crotonaldehyde:
The overall aldol condensation of acetaldehyde with acetaldehyde is then:
2 CH3CHO → CH3CH=CHCHO + H2O
Since crotonaldehyde retains the aldehyde group of its acetaldehyde parents, the reactivity of this group [1] makes it just as neurotoxic, carcinogenic, and immunosuppressive. It can also participate in the oxidative peroxide cascade described for acetaldehyde:
See "Pulmonary Peroxidosis" //www.curezone.org/forums/fm.asp?i=1984937
Although a byproduct of lipid peroxidation, like malondialdehyde:
See "Lipid Peroxidation" //www.curezone.org/forums/fm.asp?i=1976523
and present in both cigarette and wood smoke, having a potential yeast to acetaldehyde [2] to crotonaldehyde pathway in the body:
makes it much more likely to surface in various disease processes [3] even without reactive oxygen species or environmental exposure. It has been found in neurodegenerative disorders [4] and is a potent irritant in chronic pulmonary disease [5].
[1] "Aldehydes" in "Reactive Group Data sheets", CAMEO Chemicals.
http://cameochemicals.noaa.gov/react/5
[2] Gainza-Cirauqui ML et al., "Production of carcinogenic acetaldehyde by Candida albicans from patients with potentially malignant oral mucosal disorders.", J Oral Pathol Med. 2012 Aug 22.
http://www.ncbi.nlm.nih.gov/pubmed/22909057
[3] "Crotonaldehyde + Disease" from: Davis AP et al., "The Comparative Toxicogenomics Database: update 2011.", Nucleic Acids Res. 2011 Jan;39(Database issue):D1067-72. Epub 2010 Sep 22.
http://ctdbase.org/detail.go?type=chem&acc=C012796&view=disease
[4] Kawaguchi-Niida M et al., "Crotonaldehyde accumulates in glial cells of Alzheimer's disease brain.", Acta Neuropathol. 2006 May;111(5):422-9. Epub 2006 Mar 15.
http://www.ncbi.nlm.nih.gov/pubmed/16538519
[5] Moretto N et al., "alpha,beta-Unsaturated aldehydes contained in cigarette smoke elicit IL-8 release in pulmonary cells through mitogen-activated protein kinases.", Am J Physiol Lung Cell Mol Physiol. 009 May;296(5):L839-48.
http://www.ncbi.nlm.nih.gov/pubmed/19286926