Is CH3COCH3 polar or nonpolar? - Polarity of Acetone
Acetone (or dimethyl ketone) is the simplest ketone, represented by the molecular formula (CH3)2CO.
The IUPAC name for acetone is 2-propanone, implying that it possesses a 3-carbon chain and the carbonyl (C=O) functional group is present at C-2.
It exists as a colorless, highly volatile and flammable liquid at r.t.p. with a characteristic pungent odor. Acetone is most frequently used as a solvent in nail polish removers and paint thinners.
Considering the immense value of acetone as a chemical compound, we have discussed a very important chemical property of acetone in this article, i.e., its polarity.
So let us find out whether acetone is polar or non-polar.
Is acetone polar or non-polar?
Acetone (CH3COCH3 or C3H6O) is a polar molecule.
It consists of a polar carbonyl (C=O) functional group sandwiched between two relatively less polar or non-polar methyl (CH3) groups.
The C=O bond is strongly polar as a high electronegativity difference of 0.89 units is present between the covalently bonded carbon and oxygen atoms.
Contrarily, the C-H bonds are slightly polar (almost non-polar as per Pauling’s electronegativity scale), as only a small electronegativity difference of 0.35 units is present between the carbon and hydrogen atoms. The C-C bonds, on the other hand, are purely non-polar.
The strong C=O dipole moment stays uncancelled in the trigonal planar shape of acetone w.r.t the C=O bonded carbon atom. A non-uniform electron cloud distribution leads to a polar acetone molecule overall (net µ > 0).
A molecule is polar if there is a non-uniform charge distribution present in it. If the charge distribution gets equally balanced in different parts, then that molecule is considered non-polar.
The following three factors mainly influence the polarity of a molecule:
Electronegativity difference between two or more covalently bonded atoms
Dipole moment
Molecular geometry or shape
Now let’s see how the above three factors make acetone a polar molecule overall.
Factors affecting the polarity of acetone
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 elemental atoms present in acetone are carbon, hydrogen and oxygen.
Carbon (C) belongs to Group IV A (or 14) of the Periodic Table of elements. Its electronic configuration is 1s2 2s2 2p2. It has a total of 4 valence electrons which means it is still deficient in 4 more electrons in order to gain a stable octet electronic configuration.
Hydrogen (H) lies at the top of the Periodic Table in Group I A (or 1). Its electronic configuration is 1s1, which implies that it lacks 1 more electron to complete its duplet.
Oxygen (O) belongs to Group VI A (or 16). Its electronic configuration is 1s2 2s2 2p4. It has a total of 6 valence electrons. It thus needs 2 more electrons in order to complete its octet.
Atom
Electronic configuration
Valence electrons
Hydrogen (1H)
1s1
1
Carbon (6C)
1s2 2s2 2p2
4
Oxygen (8O)
1s2 2s2 2p4
6
The Lewis dot structure of acetone shows a C=O bond sandwiched between two methyls (CH3) groups. All 4 valence electrons of each C-atom get consumed in covalent bonding; thus, there is no lone pair of electrons on any one C-atom in this structure.
In contrast, only 2 valence electrons of the O-atom, out of the 6 initially available, get consumed in covalent bonding; hence, this O-atom possesses 2 lone pairs, respectively.
In this way, all the C-atoms and the O-atom complete their octet via chemical bonding and lone pairs in acetone while each H-atom attains a complete duplet electronic configuration.
As per Pauling’s electronegativity scale, a polar covalent bond is formed between two dissimilar atoms having an electronegativity difference between 0.4 to 1.6 units.
A C-C bond is purely non-polar as zero or no electronegativity difference is present between two identical C-atoms. Thus, the covalently-bonded electron cloud stays equally shared between the two carbon atoms.
In a C-H bond, only a small electronegativity difference of 0.35 units is present between a carbon (E.N = 2.55) and a hydrogen (E.N = 2.20) atom. It is less than 0.4 units; therefore, all the C-H bonds are considered almost non-polar or weakly polar.
In contrast, a high electronegativity difference of 0.89 units exists between a carbon and an oxygen (E.N = 3.44) atom. Thus, the C=O bond present in acetone is strongly polar.
Oxygen, being strongly electronegative, not only attracts the C=O electron cloud largely towards itself but also pulls the C-H bonded electrons to a great extent.
Consequently, the O-atom gains a partial negative charge (δ–), while the corresponding covalently bonded C and H-atoms obtain partial positive charges (δ+) in acetone, as shown below.
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 acetone, the strong dipole moments of the C=O bond point from Cδ+ to Oδ–while the small dipole moment of each C-H bond is directed from Hδ++ to Cδ+.
Refer to the figure drawn below.
Molecular geometry
According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, acetone is an AX3-type molecule w.r.t the C=O bonded carbon atom.
To one carbon atom at the center (A), three bond pairs (X) are attached, i.e., 1 O-atom and 2 other C-atoms. Thus, the shape of acetone w.r.t C=O bonded carbon atom is trigonal planar.
Conversely, the shape of acetone w.r.t the other two C-atoms is tetrahedral.
There is no competitor against the strong C=O dipole moment in the trigonal planar molecular shape. Thus, the strong C=O dipole moment stays uncancelled.
The charged electron cloud stays non-uniformly spread over the molecule, lending it a resultant dipole moment value. Hence, acetone (CH3COCH3) is overall polar (net µ = 2.8 D).
Difference between polar and nonpolar?
Polar molecule
Non-polar molecule
Atoms must have a difference in
electronegativity
Atoms may have the same or different electronegativity values.
Unequal charge distribution overall
Equal charge distribution overall
Net dipole moment greater than zero
Net dipole moment equals to zero.
Examples include water (H2O), ethanol (CH3CH2OH), methanol (CH3OH), acetone (C3H6O), etc.
Examples include oxygen (O2), nitrogen (N2), methane (CH4), n-hexane (C6H14), benzene (C6H6), ethylene glycol (C2H6O2), etc.
FAQ
Is acetone polar? If so, why?
Acetone is a polar molecule due to the presence of a polar carbonyl (C=O) functional group in it.
The three main types of covalent chemical bonds in an acetone molecule are C-C, C-H and C=O.
The C-C bonds are non-polar, while the C-H bonds are very weakly polar.
The C=O bond is strongly polar, having an electronegativity difference of 0.89 units between the bonded atoms.
The strong C=O dipole moment stays uncancelled in the trigonal planar shape of acetone w.r.t C=O bonded carbon atom. An unbalanced electron cloud distribution leads to an overall polar acetone molecule (net µ = 2.8 D).
What makes acetone a really good solvent? What allows it to dissolve both polar and non-polar molecules?
Like dissolves like. Acetone is composed of a polar C=O group that develops strong dipole-dipole attractions with polar molecules.
It also possesses two non-polar CH3 groups that develop attractive forces with non-polar molecules and get them solubilized.
In this way, acetone is considered a good solvent for dissolving complex samples containing both polar and non-polar molecules.
Which is more polar, acetaldehyde or acetone?
Acetone (CH3COCH3) is more polar than acetaldehyde (CH3CHO).
Alkyl groups are electron-donating or electron-pushing in nature. Two CH3 groups are attached, one on either side of C=O in acetone, while the C=O group in acetaldehyde is surrounded by one CH3 group only.
The O-atom in acetone attracts the shared electron cloud away from each methyl group and gains a stronger partial negative charge, leading to a higher net dipole moment value and, thus, a more strongly polar acetone molecule.
Which is more polar, acetone or ethyl acetate?
Ethyl acetate is an ester containing a polar C-O and C=O bond. Contrarily, acetone is a ketone possessing only one strongly polar C=O bond.
The C-O and C=O dipole moments in ethyl acetate stay uncancelled, resulting in a polar ethyl acetate molecule overall.
The net polarity effect of two strongly polar carbon-oxygen bonds is higher as opposed to a single carbon-oxygen bond. Hence, ethyl acetate is more polar than acetone.
What makes methanol more polar than acetone?
A high electronegativity difference of 1.24 units exists between the covalently bonded oxygen and hydrogen atoms in an O-H bond. 1.24 units > 0.89 units, the electronegativity difference between a carbon and oxygen atom in a C=O bond.
Thus, the hydroxyl (OH) functional group present in methanol is more strongly polar than the carbonyl (C=O) group of acetone, which is why methanol is more polar than acetone.
Summary
Acetone (2-propanone) is a polar molecule.
It comprises a polar carbonyl (C=O) functional group sandwiched between two methyls (CH3) groups.
The C-C and C-H bonds present in acetone are non-polar and weakly polar, respectively.
The C=O bond present in acetone is strongly polar as per an electronegativity difference of 0.89 units between the bonded atoms.
The strong C=O dipole moment does not get canceled in the trigonal planar shape of the molecule w.r.t the central C-atom. Thus, acetone is overall polar (net µ = 2.8 D).
References
Gizmo News. ‘Is Acetone Polar? Properties, Uses, Molecular Structure. By Brian Luoma (January 31st, 2020). https://www.gizmoplans.com/is-acetone-polar/
toppr answr. ’Is acetone a protic or an aprotic polar solvent?’’. https://www.toppr.com/ask/question.is-acetone-a-protic-or-an-aprotic-polar-solvent/
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