Is ketone more polar than alcohol? – (Alcohol vs Ketone polarity)
Ketones and alcohols are two extremely valuable members of the organic chemistry family. However, both are quite different from each other in terms of their functionality, structure, properties, and polarity.
Ketone is a carbonyl compound containing a C=O group between two alkyl chains of the same or different lengths. Thus, it is represented by a general formula R’-CO-R.
On the other hand, alcohols are organic compounds containing one or more hydroxyl (OH) functional groups attached to the alkyl chain. Thus, the general formula for an alcohol molecule is R-CH2-OH.
In this article, we will explore the polarity of ketones vs. alcohols, answering this very pertinent question i.e., which one is more polar? Ketone or alcohol? So, let’s begin!
No. Ketone is not more polar than alcohol. Rather, alcohol molecules are generally more polar than ketones of the corresponding chain lengths.
Alcohols are more polar due to the presence of an extremely polar hydroxyl (OH) functional group in their structure.
In contrast, the polarity of a ketone is accredited to the polar carbonyl (C=O) group present in it.
However, an O-H bond is more polar than a C=O bond as a higher electronegativity difference of 1.24 units is present between the covalently bonded oxygen and hydrogen atoms as opposed to an electronegativity difference of 0.89 units between the bonded atoms in C=O.
The strong O-H and C=O dipole moments stay uncancelled due to the bent shape of alcohol or a ketone w.r.t the O-atom.
However, a stronger O-H dipole moment leads to a higher net dipole moment value in alcohols as compared to ketones. Consequently, an alcohol such as ethanol (CH3CH2OH) is more polar than a ketone, for instance, acetone (CH3-CO-CH3).
The following three factors mainly influence the polarity of a molecule:
The electronegativity difference between two or more covalently bonded atoms
Dipole moment
Molecular geometry or shape
Now, let’s see how these factors contribute to making a ketone more polar than an alcohol.
Factors affecting the polarity of ketone and alcohol
Electronegativity
It is defined as the ability of an elemental atom to attract a shared pair of electrons from a covalent chemical bond.
Electronegativity increases across a period in the Periodic Table while it decreases down the group.
Greater the electronegativity difference between bonded atoms in a molecule, the higher the bond polarity.
The three main types of elemental atoms present in both ketones and alcohols are carbon (C), hydrogen (H), and oxygen (O).
The general structure of a ketone is shown below.
It consists of three main types of covalent chemical bonds, i.e., C-C, C-H and C=O.
A C-C single covalent bond is purely non-polar as it is formed between two identical carbon atoms having a zero or no electronegativity difference. The electron cloud stays uniformly shared between two C-atoms.
A small electronegativity difference of 0.35 units exists between a carbon (E.N = 2.55) and a hydrogen (E.N = 2.20) atom in a C-H bond. It is less than 0.4 units which is required for a covalent chemical bond to be polar, as per Pauling’s electronegativity scale.
Therefore, the C-H bonds present in a ketone are very weakly polar (almost non-polar).
In contrast, a higher electronegativity difference of 0.89 units is present between a carbon (E.N = 2.55) and an oxygen (E.N = 3.44) atom in the C=O bond. So, the C=O bond is strongly polar in ketones.
Oxygen, being more electronegative, attracts the C=O bonded electrons largely towards itself and gains a partial negative charge (δ–) while the corresponding C-atom obtains a partial positive charge (δ+), respectively.
The formation of the oppositely charged poles induces polarity in a ketone.
Now let us see the general structure of an alcohol molecule, drawn below.
It consists of non-polar C-C bonds, weakly polar C-H bonds in the alkyl chain, followed by a polar C-O bond.
However, a strongly polar O-H bond is also present in this structure, owing to a high electronegativity difference of 1.24 units between the covalently bonded oxygen and hydrogen atoms.
Oxygen, being strongly electronegative, attracts the O-H shared electron cloud to a large extent and gains a partial negative charge (δ–) while the corresponding H-atom obtains a partial positive charge (δ+), respectively.
The greater the electronegativity difference, the more strongly the more electronegative atom pulls the shared electron cloud towards itself.
Dipole moment
Dipole moment (μ) is a vector quantity that points from the positive pole to the negative pole of a bond or a molecule.
It is mathematically calculated as a product of the magnitude of charge (Q) and charge separation (r). The dipole moment is expressed in a unit called Debye (D).
The dipole moment of a polar covalent bond conventionally points from the positive center to the center of the negative charge.
In both ketones and alcohols, the small C-H dipole moments point from Hδ++ to Cδ+.
Similarly, the strong C=O and C-O dipole moments in ketone and alcohols, respectively, are directed from Cδ+ to Oδ–.
The alcohols have an additional, stronger O-H dipole moment that points from Hδ+ to Oδ–, as shown below.
Molecular geometry
The molecular geometry or shape of both a ketone as well as an alcohol molecule w.r.t each C-atom is tetrahedral. Contrarily, the O-atoms carry 2 lone pairs of electrons each; thus, the shape of a ketone or an alcohol w.r.t the O-atom is bent, angular or V-shaped.
It is due to this asymmetric bent shape w.r.t the O-atom that the dipole moments of individually polar bonds stay uncancelled in both types of molecules. Thus, ketone and alcohol are both polar molecules (net µ > 0).
The only factor differentiating between the two in terms of polarity is the presence of a C=O or an O-H bond.
Alcohols containing O-H bonds possess higher net dipole moment values as compared to ketones.
Alcohols can also form hydrogen bonding with other polar solvents, such as H2O, further enhancing their polarity effect. Similarly, alcohols may experience intramolecular H-bonding, as shown below.
In conclusion, alcohol is typically more polar than ketone.
FAQ
Is ketone more polar than alcohol?
No.A ketone is generally less polar than an alcohol of corresponding chain length.
The polarity of a ketone is due to the polar C=O group having an electronegativity difference of 0.89 units between the bonded atoms.
The polarity of alcohol is due to the polar O-H group having an electronegativity difference of 1.24 units between the bonded atoms.
1.24 units > 0.89 units. So, the O-H bond is more polar than a C=O bond.
Higher bond polarity leads to a greater net dipole moment value. Consequently, alcohols (net µ > > 0) are more polar than ketones (net µ > 0).
Why are ketones more polar than aldehydes?
A C=O group is sandwiched between two alkyl chains in a ketone, while the general structure of an aldehyde is R-CHO. The alkyl (R) group is electron-donating.
Oxygen, being strongly electronegative, pulls the C-H electron cloud of nearby alkyl chains in addition to attracting the C=O bonded electrons.
Two R-groups are attached to the C=O bond in a ketone, so it is generally more polar than an aldehyde having one R-group only.
Which ketone is more polar? – butanone or acetone?
Acetone is slightly more polar than 2-butanone (also known as ethyl methyl ketone or MEK). A longer alkyl chain (4 C-chain) in butanone means a greater number of non-polar and weakly polar C-C and C-H bonds.
However, both contain only one strongly polar C=O bond. The net dipole moment effect is reduced due to a longer non-polar alkyl chain; thus, acetone (net µ = 2.88 D) is comparatively more polar than 2-butanone (net µ = 2.69 D).
Why alcohol is more polar? – ethanol or methanol?
Methanol (CH3OH), with a smaller molecular size, is more polar than ethanol (CH3CH2OH).
The C-C bonds possess zero or no dipole moment values, while the C-H bonds possess very small dipole moments.
The strong O-H dipole moment of ethanol gets canceled to a greater extent (although not fully canceled) with five small C-H dipole moments, which results in a smaller net dipole moment value as compared to the resultant dipole moment of methanol, in which the central C-atom is surrounded by 3 H-atoms and no other C-atom.
Which is a more polar solvent? Ethanol or acetone?
Ethanol is definitely more polar as a molecule as well as a solvent as compared to acetone due to a strongly polar O-H group present in it.
It is due to this O-H group that multiple ethanols (CH3CH2OH) molecules can form H-bonding with one another, increasing their boiling point and also their overall polarity effect as a solvent.
Contrarily, no such intramolecular H-bonding is possible among acetone molecules.
Summary
Ketones and alcohols are both polar in nature.
Alcohols are generally more polar than ketones of corresponding chain length.
The higher polarity of an alcohol molecule is accredited to the presence of a strongly polar O-H group, enabling it to form an H-bonding with other polar molecules.
An O-H bond (in alcohols) is more polar than the C=O bond (in ketones) due to a higher electronegativity difference of 1.24 units between the bonded atoms in the former.
A stronger O-H dipole moment leads to a higher resultant dipole moment value, thus a strongly polar alcohol molecule as compared to moderately polar ketones.
Vishal Goyal is the founder of Topblogtenz, a comprehensive resource for students seeking guidance and support in their chemistry studies. He holds a degree in B.Tech (Chemical Engineering) and has four years of experience as a chemistry tutor. The team at Topblogtenz includes experts like experienced researchers, professors, and educators, with the goal of making complex subjects like chemistry accessible and understandable for all. A passion for sharing knowledge and a love for chemistry and science drives the team behind the website. Let's connect through LinkedIn: https://www.linkedin.com/in/vishal-goyal-2926a122b/
Topblogtenz is a website dedicated to providing informative and engaging content related to the field of chemistry and science. We aim to make complex subjects, like chemistry, approachable and enjoyable for everyone.
