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  Properties of Aldehydes & Ketones Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Properties of Aldehydes and Ketones

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A contrast of the reactivity and properties of aldehydes and ketones with those of the alkenes is warranted, because both have a double bond functional group. Due to the greater electronegativity of oxygen, the carbonyl group is polar, and ketones and aldehydes have larger molecular dipole moments (D) than do alkenes. The resonance structures on the right demonstrate this polarity and the comparative dipole moments of formaldehyde, another ketones and aldehydes confirm the stabilizing effect that alkyl substituents have on carbocations (the larger the dipole moment the greater the polar character of the carbonyl group). So, we suppose, that aldehydes and ketones will have higher boiling points than identical sized alkenes. Additionally, the existence of oxygen with its non-bonding electron pairs make aldehydes and ketones hydrogen-bond acceptors and have to increase their water solubility relative to hydrocarbons. Particular instances of these relationships are provided in the table below.

Compound

Mol. Wt.

Boiling Point

Water 
Solubility

(CH3)2C=CH2

56

-7.0 ºC

0.04 g/100

(CH3)2C=O

58

56.5 ºC

infinite

CH3CH2CH2CH=CH2

70

30.0 ºC

0.03 g/100

CH3CH2CH2CH=O

72

76.0 ºC

7 g/100

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96

103.0 ºC

insoluble

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98

155.6 ºC

5 g/100

 

 

 

 

 

 

 

 

 

 

 

 

The polarity of the carbonyl group also has a profound influence on its chemical reactivity, as compared with the non-polar double bonds of alkenes. So, reversible addition of water to the carbonyl function is fast, where water addition to alkenes is immeasurably slow in the absence of a strong acid catalyst. Curiously, relative bond energies affect the thermodynamics of such type of addition reactions in the opposite sense.

The C=C of alkenes has an average bond energy of 146 kcal/mole. Because a C-C σ-bond has bond energy of 83 kcal/mole, the π-bond energy might be estimated at 63 kcal/mole. The C=O bond energy of a carbonyl group, alternatively, changes with its location, as follows:

    H2C=O     170 kcal/mole    
    RCH=O    175 kcal/mole    
    R2C=O     180 kcal/mole    

The C-O σ-bond establishes to have average bond energy of 86 kcal/mole. Subsequently, with the exception of formaldehyde, carbonyl function of aldehydes and ketones has a π-bond energy greater than that of the sigma-bond, in difference to the pi-sigma relationship in C=C. This proposes that addition reactions to carbonyl groups should be thermodynamically disfavored, like is the case for the addition of water. All of this is shortened in the diagram below (ΔHº values are for the addition reaction).

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Even though the addition of water to an alkene is exothermic and provides a stable product (an alcohol), the uncatalyzed reaction is very slow because of a high activation energy . The opposite reaction (dehydration of an alcohol) is even slower, and due to the kinetic difficulty, both reactions are practical only in the existence of a strong acid.

In difference, both the endothermic addition of water to a carbonyl function and the exothermic elimination of the water by the resulting geminal-diol are fast. The carbonyl group's inherent polarity, together with its increased basicity (as compared with alkenes), lowers transition state energy for both reactions, with a resulting increase in rate. Bases and Acids catalyze both the addition and elimination of water. Evidence that fast and reversible addition of water to carbonyl compounds takes place is provide by experiments using isotopically labelled water. A quock exchange of the oxygen isotope takes place, If a carbonyl reactant composed of 16O (colored blue above) is treated with water incorporating the 18O isotope (colored red in the diagram). This can only be described by the addition-elimination technique shown here.

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