I am unsure with regards to the specific reason why alkanes are non-polar as I have heard different explanations. My teacher said that it is because, although there is a difference in electronegativity between carbon and hydrogen atoms, the carbon is shielded by the hydrogen atoms from reacting with the external environment and therefore, since only the hydrogen can react, it is non-polar (something along the lines of it is the difference between delta + and delta - ends that actually make a molecule polar). From searching on the internet, however, I have found the explanation that it is because the difference in electronegativity is so small (0.3) that it is non-polar, which makes sense, however it contradicts what my textbook states: That only bonds between elements with equal electronegativity are non-polar. Can someone clear this up for me? Thank you!
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8$\begingroup$ It is just a matter of semantics. The difference in electronegativity between C and H is just so small that, for all intents and purposes, the bond (and the molecule) is practically non-polar. $\endgroup$– orthocresolDec 7, 2015 at 12:48
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1$\begingroup$ In addition, unsubstituted, symmetrical alkanes, e.g. methane, are even less polar (though there is still a small instantaneous dipole moment). See butane.chem.uiuc.edu/cyerkes/Chem104ACSpring2009/… $\endgroup$– DrMoishe PippikDec 8, 2015 at 1:58
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$\begingroup$ so it does not have anything too do with the fact that carbon is 'on the inside' and therefore cant interact with the outside environment that it is not polar? $\endgroup$– frostedcakeDec 8, 2015 at 17:10
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$\begingroup$ @frostedcake No, it doesn't. Also, there is quite a set of polar hydricarbons like azulene $\endgroup$– permeakraJan 14, 2016 at 17:52
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1$\begingroup$ This highlights a common theme throughout much of science. Very rarely are two things exactly equal. The question to ask is, "are they 'equal enough' that we can't really tell a difference between them?" In this case, the electronegativities of $\ce C$ and $\ce H$ are "equal enough" to make alkanes non-polar. $\endgroup$– hBy2PyJan 14, 2016 at 19:27
3 Answers
There are two reasons why molecules are polar or not: the symmetry of the molecule and the polarity of the bonds in it.
Bonds can be thought to have electric dipoles if the elements in the bonds are different. Bonds between more polar elements will have stronger dipoles. But, in a molecule with many bonds, you need to know the overall structure to know whether the molecule is polar or not. Imagine taking all the individual bond dipoles and adding them up (as vectors, in other words taking account of the direction they point as well as the size of the dipole).
So polar bonds are not enough to make a molecule polar. Carbon tetrachloride has 4 fairly polar bonds but is non-polar because the dipoles in each bond cancel each other out because of symmetry. Many simple hydrocarbons are non-polar for this reason: they have enough symmetry that any polarity in their C-H bonds cancel out. Many simple hydrocarbons are too symmetric to have any polarity, but some are not: isobutane, HC(CH3)3 is polar (dipole moment 0.13 D) because it isn't symmetric and the slight dipoles in its C-H bonds don't cancel out. But compare to chloromethane which is asymmetric and has a very polar bond: it is fairly polar (dipole moment 1.9 D) as it is dominated by just one polar C-Cl bond.
But even the non-symmetric hydrocarbons are not very polar compared to things like chlorinated hydrocarbons so are often grouped with completely non-polar compounds.
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$\begingroup$ This is the correct answer, addressing both the (roughly) non-polarity of the bonds themselves and the non-polarity of the overall molecule. $\endgroup$ Oct 29, 2021 at 18:32
It is because of the very small difference in electronegativity between carbon and hydrogen.This is the reason why alkanes are very less acidic,do not show electrophillic substitutions etc.
Strictly speaking, everything is polar unless it has a perfect spherically symmetric charge distribution, which in ordinary chemical settings means noble gas atoms. Otherwise, where there is not a dipole moment there can be higher order polarity and it can impact solubility and reactivity. The quadrupole in carbon dioxide is one such example.
However, in alkanes all the electrical polarities, not just dipoles, are so small that they have little effect on the properties of the material as a solvent or solute, so they act as a fully nonpolar material would.
