INTRODUCTION TO CARBONYL COMPOUNDS AND THEIR PHYSICAL PROPERTIES

Carbonyl Compounds

The carbonyl compounds include the aldehydes and ketones. The carbonyl functional group (C = O) consists of a carbon atom double‐bonded to an oxygen atom. The carbonyl group is strongly polar (Cδ+ = O δ), and this has a marked effect on the physical and chemical properties of the carbonyl compounds.

Carbonyl compounds are very common in our everyday lives. The poisonous aldehyde‐based compound used in slug pellets and the ketones used as solvents for nail polish remover and for a wide variety of industrial processes have physical and chemical properties determined by the carbonyl group.

The characteristic odor of cherries is due to benzaldehyde. Aldehydes and ketones comprise a number of interesting everyday compounds. Aldehydes such as vanillin and cinnamaldehyde (odor of cinnamon) impart odors familiar to all of us. The odor of garlic is produced by ketones.

The horrible millipede bite produces its sting due to the presence of benzaldehyde and hydrogen cyanide in the fluid released. The drug cortisone is a ketone used to relieve individuals with rheumatism. Many cosmetics contain carbonyl compounds.\

Aldehydes and ketones

Aldehydes and ketones are both homologous series containing the carbonyl group. Both organic families have the general formula CnH2nO.

The position of the C=O functional group in the carbon chain marks the difference between aldehydes and ketones.

The aldehydes have the carbonyl group at the end of the carbon chain and have the general structure RCHO. The carbonyl group of the ketones is not at the end of the carbon chain. Its position within the molecule varies making ketones to exhibit positional isomerism.

Formaldehyde is the simplest aldehyde. It was used as a preservative but is used less now because it is thought to be carcinogenic. It is the starting material for many reactions. Acetone is the simplest ketone. It is commonly used as an industrial solvent. Nail polish remover contains acetone.

Examples for aldehydes and ketones Aldehydes and ketones are a group of compounds containing the carbonyl group, C=O.

Structure of the carboxyl acid group

Carboxylic acids are organic compounds which incorporate a carboxyl functional group, CO2H.  The name carboxyl comes from the fact that a carbonyl and a hydroxyl group are attached to the same carbon.

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The carbon and oxygen in the carbonyl are both sp2 hybridized which give a carbonyl group a basic trigonal shape.  The hydroxyl oxygen is also sp2 hybridized which allows one of its lone pair electrons to conjugate with the pi system of the carbonyl group.  This make the carboxyl group planar an can represented with the following resonance structure.

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Carboxylic acids are named such because they can donate a hydrogen to produce a carboxylate ion.  The factors which affect the acidity of carboxylic acids will be discussed later.

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Physical Properties of Some Carboxylic Acids

 

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The table at the beginning gave the melting and boiling points for a homologous group of carboxylic acids having from one to ten carbon atoms. The boiling points increased with size in a regular manner, but the melting points did not. Unbranched acids made up of an even number of carbon atoms have melting points higher than the odd numbered homologs having one more or one less carbon. This reflects differences in intermolecular attractive forces in the crystalline state. In the table of fatty acids we see that the presence of a cis-double bond significantly lowers the melting point of a compound. Thus, palmitoleic acid melts over 60º lower than palmitic acid, and similar decreases occur for the C18 and C20 compounds. Again, changes in crystal packing and intermolecular forces are responsible.

The factors that influence the relative boiling points and water solubilities of various types of compounds were discussed earlier. In general, dipolar attractive forces between molecules act to increase the boiling point of a given compound, with hydrogen bonds being an extreme example. Hydrogen bonding is also a major factor in the water solubility of covalent compounds To refresh your understanding of these principle. The following table lists a few examples of these properties for some similar sized polar compounds (the non-polar hydrocarbon hexane is provided for comparison).

 

 

Physical Properties of Some Organic Compounds

FormulaIUPAC NameMolecular WeightBoiling PointWater Solubility
CH3(CH2)2CO2Hbutanoic acid88164 ºCvery soluble
CH3(CH2)4OH1-pentanol88138 ºCslightly soluble
CH3(CH2)3CHOpentanal86103 ºCslightly soluble
CH3CO2C2H5ethyl ethanoate8877 ºCmoderately soluble
CH3CH2CO2CH3methyl propanoate8880 ºCslightly soluble
CH3(CH2)2CONH2butanamide87216 ºCsoluble
CH3CON(CH3)2N,N-dimethylethanamide87165 ºCvery soluble
CH3(CH2)4NH21-aminobutane87103 ºCvery soluble
CH3(CH2)3CNpentanenitrile83140 ºCslightly soluble
CH3(CH2)4CH3hexane8669 ºCinsoluble

 

The first five entries all have oxygen functional groups, and the relatively high boiling points of the first two is clearly due to hydrogen bonding. Carboxylic acids have exceptionally high boiling points, due in large part to dimeric associations involving two hydrogen bonds. A structural formula for the dimer of acetic acid is shown here. When the mouse pointer passes over the drawing, an electron cloud diagram will appear. The high boiling points of the amides and nitriles are due in large part to strong dipole attractions, supplemented in some cases by hydrogen bonding.

 

Acidity of Carboxylic Acids

The pKa ‘s of some typical carboxylic acids are listed in the following table. When we compare these values with those of comparable alcohols, such as ethanol (pKa = 16) and 2-methyl-2-propanol (pKa = 19), it is clear that carboxylic acids are stronger acids by over ten powers of ten! Furthermore, electronegative substituents near the carboxyl group act to increase the acidity.

CompoundpKaCompoundpKa
HCO2H3.75CH3CH2CH2CO2H4.82
CH3CO2H4.74ClCH2CH2CH2CO2H4.53
FCH2CO2H2.65CH3CHClCH2CO2H4.05
ClCH2CO2H2.85CH3CH2CHClCO2H2.89
BrCH2CO2H2.90C6H5CO2H4.20
ICH2CO2H3.10p-O2NC6H4CO2H3.45
Cl3CCO2H0.77p-CH3OC6H4CO2H4.45

Why should the presence of a carbonyl group adjacent to a hydroxyl group have such a profound effect on the acidity of the hydroxyl proton? To answer this question we must return to the nature of acid-base equilibrium and the definition of pKa , illustrated by the general equations given below. These relationships were described in an previous section of this text.

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We know that an equilibrium favors the thermodynamically more stable side, and that the magnitude of the equilibrium constant reflects the energy difference between the components of each side. In an acid base equilibrium the equilibrium always favors the weaker acid and base (these are the more stable components). Water is the standard base used for pKameasurements; consequently, anything that stabilizes the conjugate base (A:(–)) of an acid will necessarily make that acid (H–A) stronger and shift the equilibrium to the right. Both the carboxyl group and the carboxylate anion are stabilized by resonance, but the stabilization of the anion is much greater than that of the neutral function, as shown in the following diagram. In the carboxylate anion the two contributing structures have equal weight in the hybrid, and the C–O bonds are of equal length (between a double and a single bond). This stabilization leads to a markedly increased acidity, as illustrated by the energy diagram displayed by clicking the “Toggle Display” button.

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The resonance effect described here is undoubtedly the major contributor to the exceptional acidity of carboxylic acids. However, inductive effects also play a role. For example, alcohols have pKa‘s of 16 or greater but their acidity is increased by electron withdrawing substituents on the alkyl group. The following diagram illustrates this factor for several simple inorganic and organic compounds (row #1), and shows how inductive electron withdrawal may also increase the acidity of carboxylic acids (rows #2 & 3). The acidic hydrogen is colored red in all examples.

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Water is less acidic than hydrogen peroxide because hydrogen is less electronegative than oxygen, and the covalent bond joining these atoms is polarized in the manner shown. Alcohols are slightly less acidic than water, due to the poor electronegativity of carbon, but chloral hydrate, Cl3CCH(OH)2, and 2,2,2,-trifluoroethanol are significantly more acidic than water, due to inductive electron withdrawal by the electronegative halogens (and the second oxygen in chloral hydrate). In the case of carboxylic acids, if the electrophilic character of the carbonyl carbon is decreased the acidity of the carboxylic acid will also decrease. Similarly, an increase in its electrophilicity will increase the acidity of the acid. Acetic acid is ten times weaker an acid than formic acid (first two entries in the second row), confirming the electron donating character of an alkyl group relative to hydrogen, as noted earlier in a discussion of carbocation stability. Electronegative substituents increase acidity by inductive electron withdrawal. As expected, the higher the electronegativity of the substituent the greater the increase in acidity (F > Cl > Br > I), and the closer the substituent is to the carboxyl group the greater is its effect (isomers in the 3rd row). Substituents also influence the acidity of benzoic acid derivatives, but resonance effects compete with inductive effects. The methoxy group is electron donating and the nitro group is electron withdrawing (last three entries in the table of pKa values).

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