File Name: physical properties of aldehydes and ketones .zip
Learning Objectives. The carbon-to-oxygen double bond is quite polar, more polar than a carbon-to-oxygen single bond. The electronegative oxygen atom has a much greater attraction for the bonding electron pairs than does the carbon atom. The carbon atom has a partial positive charge, and the oxygen atom has a partial negative charge:. In aldehydes and ketones, this charge separation leads to dipole-dipole interactions that are great enough to significantly affect the boiling points. Formaldehyde is a gas at room temperature. Most other common aldehydes are liquids at room temperature.
The boiling point of aldehydes and ketones is higher than that of non-polar compounds hydrocarbons but lower than those of corresponding alcohols and carboxylic acids as aldehydes and ketones do not form H-bonds with themselves. The lower members up to 4 carbons of aldehydes and ketones are soluble in water due to H-bonding. The higher members do not dissolve in water because the hydrocarbon part is larger and resists the formation of hydrogen bonds with water molecules. Both aldehydes and ketones contain carbonyl group, therefore they undergo same reactions like nucleophilic addition reactions, oxidation, reduction, halogenation etc. Aromatic aldehydes and ketones exhibit electron donating resonance which increases the electron density on the carbonyl carbon. Because of this reason, the carbonyl carbon becomes less electrophilic, and hence is less susceptible to nucleophilic attack.
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The only structural difference between hydrocarbons and aldehydes is the presence in the latter of the carbonyl group, and it is this group that is responsible for the differences in properties, both physical and chemical. This gives the oxygen a partial negative charge and the carbon a partial positive charge. The negative end of one polar molecule is attracted to the positive end of another polar molecule, which may be a molecule either of the same substance or of a different substance. The polarity of the carbonyl group notably affects the physical properties of melting point and boiling point , solubility, and dipole moment. Hydrocarbons, compounds consisting of only the elements hydrogen and carbon, are essentially nonpolar and thus have low melting and boiling points. The melting and boiling points of carbonyl-containing compounds are considerably higher. The reason for the large difference is that polar molecules have a greater attraction for each other than do nonpolar molecules, requiring more energy—and thus a higher temperature—to separate them, which must occur if compounds are to melt or boil.
This page explains what aldehydes and ketones are, and looks at the way their bonding affects their reactivity. It also considers their simple physical properties such as solubility and boiling points. Aldehydes and ketones are simple compounds which contain a carbonyl group - a carbon-oxygen double bond. They are simple in the sense that they don't have other reactive groups like -OH or -Cl attached directly to the carbon atom in the carbonyl group - as you might find, for example, in carboxylic acids containing -COOH. In aldehydes, the carbonyl group has a hydrogen atom attached to it together with either a second hydrogen atom or, more commonly, a hydrocarbon group which might be an alkyl group or one containing a benzene ring. For the purposes of this section, we shall ignore those containing benzene rings. Notice that these all have exactly the same end to the molecule.
Solubility: Aldehydes and ketones are soluble in water but their solubility decreases with increase in the length of the chain. Methanal, ethanal and propanone are.
The addition of hydrogen cyanide and of sodium hydrogensulphite sodium bisulphite to aldehydes and ketones. The reduction of aldehydes and ketones using sodium tetrahydridoborate III or lithium tetrahydridoaluminate III sodium borohydride or lithium aluminium hydride. The reactions of aldehydes and ketones with Grignard reagents as a way of making complicated alcohols. Covers the main ways of distinguishing between aldehydes and ketones using, for example, Tollens' reagent, Fehling's solution or Benedict's solution. Looks at the test for aldehydes and ketones using 2,4-dinitrophenylhydrazine Brady's reagent , plus a quick look at some similar reactions.
The carbon atom of this group has two remaining bonds that may be occupied by hydrogen or alkyl or aryl substituents. If at least one of these substituents is hydrogen, the compound is an aldehyde. If neither is hydrogen, the compound is a ketone. The IUPAC system of nomenclature assigns a characteristic suffix to these classes, al to aldehydes and one to ketones. Since an aldehyde carbonyl group must always lie at the end of a carbon chain, it is by default position 1, and therefore defines the numbering direction. A ketone carbonyl function may be located anywhere within a chain or ring, and its position is given by a locator number. Chain numbering normally starts from the end nearest the carbonyl group.
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