Polarity of aldehyde have been extensively studied and several understandings have been put on to know whether are aldehydes polar and is being used in various applications.
Aldehydes are polar compounds due to the presence of the carbonyl carbon(R-CHO) functional group in it in which the oxygen is more electronegative than that of the carbonyl carbon attracting the electrons towards itself making it a polar compound.
Due to the presence of this more electronegative oxygen atom to carbon atom in the carbonyl functional group making aldehyde a polar compound. Since both aldehyde(R-CHO) and ketone(R-CO-R’) have the carbonyl functional group in their structure but due to the difference in the presence of alkyl group in both aldehyde and ketone making ketone more polar.
As you study about the polarity of aldehydes i.e., are aldehydes polar and its relation to ketone compounds the more you read the more you will learn and the more you understand and new concepts about its polarity can be formed and different kind of applications can be developed through it so, lets discuss about the facts in detail.
Are aldehydes polar?
The question for ‘are aldehydes polar or not’ is it is that yes, Aldehydes are the polar compounds having carbonyl compounds with general structure RCH=O group in it.
we represent the structure of aldehyde as;
The carbonyl carbon in aldehyde is sp2 hybridized and is linked to 3 other atoms by strong sigma bonds. The remaining p orbitals of carbonyl carbon overlap with the p orbital of oxygen atom to form pi bonds.
The ‘R’ in aldehyde structure(carbonyl compound structure) can be an alkyl, aryl or an hydrogen group and depending on the group present its inductive effects keep on changing.
These bonds are in a plane and are 120º apart because they use sp2 hybrid orbitals and thus provide dipole dipole interaction.
here in the structure for aldehyde R’=H and R can be hydrogen, alkyl or an aryl group.
How are aldehydes polar?
In the C=O bond in aldehyde, the oxygen atom is more electronegative or less electropositive than the carbon atom which is more electropositive or less electronegative.
It is because of that the electron density in it are drawn more towards the more electronegative oxygen atom and thus causing a negative charge in oxygen atom and a partial positive charge on carbon atom leading the carbon atom to become electron deficient and oxygen to become electron rich thus making the compound more polar
the electrons involved in pi bond formation are drawn towards the more electronegative oxygen atom causing a delta+ charge on carbon and delta– charge on oxygen atom(the polarity of the carbonyl compound is represented by(delta). from this structure we will understand whether are aldehydes polar or not.
The carbonyl group in aldehyde is flat and because of that they can be attacked from both above and below the plane by other polar compounds and thus making it highly reactive Since the carbonyl carbon in aldehyde in electron deficient can be easily attacked by nucleophile (electron rich) leading to a bond formation.
This nucleophilic attack is catalyzed by acids because they intensify the deficiency of electrons of the carbon atom by combining its electron with oxygen then a positive charge is developed on the oxygen atom which helps in pi electron movement towards oxygen.
Are aldehydes more polar than ketones?
Aldehydes and ketones have approximately same polarity since the carbonyl group RCH=O is common in both of them ie, the aldehyde and the ketone group and because of that in most cases aldehydes and ketones have similar properties but ketones are slightly more polar than aldehyde.
The slightest polarity difference is that ketones have 2 alkyl group that releases electron density towards carbonyl group by +I effect making carbonyl group more polar.
Aldehydes do only have one alkyl group attached to carbonyl group and thus the electron density released towards carbonyl group will be less compared to ketone making it less polar.
Consequently, the destabilization of transition state resulting from ketone will be greater than that of the transition state resulting from aldehyde thus affecting the reactivity of aldehydes and ketones.
The increasing order of +I effect is given as;
the reactivity of the aldehyde and ketone compound decreases with increase in +I effect. As we move from left to right +I effect increases and reactivity decreases
The other same reason is that due to the absence of hydrogen atom in ketone compound does not undergo oxidation , as aldehyde have hydrogen atom in its carbonyl structure can easily undergo oxidation forming acids but ketones can undergo oxidation in the presence of strongly oxidizing agents.
Dipole moment and polarity;
The greater the dipole moment the larger the polar character of carbonyl group which means that is the aldehyde group as well.
Here the dipole moment value of ketone(D=2.91) is more than that of the dipole moment value of aldehyde(D=2.7) making it more polar.
The dipole dipole interaction between polar compounds is given as;
here polar compounds interact with other polar compounds. the more electronegative end (oxygen) of one carbonyl compound interact with the electropositive end (carbon atom) of another carbonyl compound.
Polarity and Boiling Point;
Because of this more polarity of ketone compared to aldehyde is also the reason why ketone has higher boiling point than that of aldehyde and then because of the presence of this 2 alkyl group in ketone making it a larger molecule than aldehyde.
Ketone will have more electrons and nuclei and thus will cause Vander Waals dispersion force which will make ketone have high boiling point
E.g., Boiling point of acetone(CH3COCH3) is 56.05ºC
Boiling point of propanal (CH3CH3CHO) 49ºC
Here the chemical formula of acetone is C3H6O and similarly the chemical formula of propanal is also C3H6O. Even their molar mass is the same that is, 58.08g/mol but due to the difference in the presence of alkyl group in both aldehyde and ketone making ketone more polar than aldehyde and thus also the more boiling point of ketone than aldehyde
As the carbon atoms number in a compound increase, Boiling Point of a compound also increase
Molecules having high polarity have greater dispersion force and thus the stronger dipole- dipole interaction leading to high boiling point
Polarity and Melting point;
Melting Point of a compound increases with increase in polarity and therefore polar compounds have high melting point and non-polar compounds have low melting point
E.g., the melting point of acetone is -95ºC
The melting point of propionaldehyde(propanal) is -80ºC
As ketone is more polar than aldehyde and therefore have high melting point
Polarity and Solubility;
Aldehydes and ketones do not have hydrogen atom on oxygen (like in acid -COOH) and so they cannot donate hydrogen but can accept hydrogen from other compounds and hence are soluble in water.
The chief characteristics of water is that it tends to exclude non polar species (hydrophilic for polar molecules or groups and hydrophobic for non-polar molecules or groups) and this is the reason why ketone is more soluble in water than aldehyde because ketone is more polar than aldehyde.
To check the solubility of aldehyde and ketone in water the condition is that they should have same number of carbon atoms and that the aldehydes and ketones having more than 5 carbon atoms are insoluble in water i.e., as the number of carbon atom increases the solubility of aldehyde and ketone in water decreases