Editing Ester (section)

== Structure and bonding ==
Esters derived from [[carboxylic acids]] and [[alcohols]] contain a [[carbonyl]] group C=O, which is a [[divalent]] group at [[Carbon|C]] atom, which gives rise to {{not a typo|120°}} C–C–O and O–C–O angles. Unlike [[amide]]s, carboxylic acid esters are structurally flexible functional groups because rotation about the C–O–C bonds has a low barrier. Their flexibility and low polarity is manifested in their physical properties; they tend to be less rigid (lower melting point) and more volatile (lower boiling point) than the corresponding [[amides]].<ref name=March>March, J. ''Advanced Organic Chemistry'' 4th Ed. J. Wiley and Sons, 1992: New York. {{ISBN|0-471-60180-2}}.</ref> The [[acid dissociation constant|p''K''<sub>a</sub>]] of the alpha-hydrogens on esters of carboxylic acids is around 25 (alpha-hydrogen is a hydrogen bound to the carbon adjacent to the [[carbonyl group]] (C=O) of carboxylate esters).<ref>{{cite web| url=http://pharmaxchange.info/press/2011/02/chemistry-of-enolates-and-enols-acidity-of-alpha-hydrogens/| title=Chemistry of Enols and Enolates – Acidity of alpha-hydrogens| date=13 February 2011}}</ref>

Many carboxylic acid esters have the potential for [[conformational isomerism]], but they tend to adopt an ''S''-''cis'' (or ''Z'') conformation rather than the ''S''-''trans'' (or ''E'') alternative, due to a combination of [[Anomeric effect#Dipole Minimization|hyperconjugation and dipole minimization]] effects. The preference for the ''Z'' conformation is influenced by the nature of the substituents and solvent, if present.<ref>{{cite journal | author=Diwakar M. Pawar | author2=Abdelnaser A. Khalil | author3=Denise R. Hooks | author4=Kenneth Collins | author5=Tijuana Elliott | author6=Jefforey Stafford | author7=Lucille Smith | author8=Eric A. Noe | title=''E'' and ''Z'' Conformations of Esters, Thiol Esters, and Amides | journal=[[Journal of the American Chemical Society]] | year=1998 | volume=120 | issue=9 | pages=2108–2112 | doi=10.1021/ja9723848| bibcode=1998JAChS.120.2108P }}</ref><ref>{{cite journal | author=Christophe Dugave | author2=Luc Demange | title=Cis−Trans Isomerization of Organic Molecules and Biomolecules: Implications and Applications | journal=[[Chemical Reviews]] | year=2003 | volume=103 | issue=7 | pages=2475–2932 | doi=10.1021/cr0104375 | pmid=12848578}}</ref> [[Lactone]]s with small rings are restricted to the ''s''-trans (i.e. ''E'') conformation due to their cyclic structure.

[[File:Ester conformers.png|center|300px]]
[[File:PhCO2MeStructure.png|thumb|left|144px|Metrical details for [[methyl benzoate]], distances in picometers.<ref>{{cite journal|author1=A. A. Yakovenko |author2=J. H. Gallegos |author3=M. Yu. Antipin |author4=A. Masunov |author5=T. V. Timofeeva |journal=Crystal Growth & Design |year=2011 |doi=10.1021/cg200547k |volume=11 |title=Crystal Morphology as an Evidence of Supramolecular Organization in Adducts of 1,2-Bis(chloromercurio)tetrafluorobenzene with Organic Esters |issue=9 |pages=3964–3978|bibcode=2011CrGrD..11.3964Y }}</ref>]]