DATABASE IB CHEMISTRY IA - Investigating the effect of structural isomerism on the boiling points of aliphatic alcohols and ethers
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This document is a database Chemistry HL IA for the IB board. It investigates the effect of structural isomerism (positional, chain, and metamerism) on the boiling points of aliphatic alcohols and ethers. This can help you get a high mark for your Chemistry course especially if you are looking to d...
Investigating the effect of structural isomerism (positional, chain and metamerism) on the boiling
points of aliphatic alcohols and ethers
Introduction
While learning about intermolecular forces in our Chemistry lessons, alcohols were taught as prime examples of
hydrogen bonding between molecules. We learned that the boiling points of alcohols are significantly higher than
hydrocarbons of comparable molar mass (e.g. alkanes or alkenes) because of the strong intermolecular hydrogen
bonding present in alcohols. Alcohols find use in our everyday lives and in industries alike; for instance isopropyl
alcohol (2-propanol) is commonly applied to the skin as an antiseptic, and is also an important component in lotions
and other cosmetics. On the other hand, its isomer propyl alcohol (1-propanol) is largely used in industries as a
solvent in pharmaceuticals and in the preparation of lacquers. Both compounds differ only by the position of the
hydroxyl group on the carbon chain and yet they have widely different physical properties and thus find different
uses. This concept led me to consider why such small changes in the structure of a molecule have a drastic impact
on its properties and how intermolecular forces play a role in it.
The boiling point of a compound depends on the intermolecular forces that exist between its molecules. Alcohols
are organic compounds characterized by the presence of a hydroxyl functional group (–OH) attached to an alkyl
group on a hydrocarbon chain. The hydroxyl group is able to form
hydrogen bonds, the strongest intermolecular forces, with hydroxyl
groups present in other alcohol molecules. When oxygen, a highly
electronegative atom, covalently bonds to a hydrogen atom within an
alcohol molecule, it induces polarity in the molecule. A partially positive
charge is formed on the hydrogen atom and a partially negative charge is
formed on the oxygen atom. As a result of the high difference in
electronegativity, the hydrogen atom in one molecule is highly
electrostatically attracted to the oxygen atom in a neighboring alcohol
molecule. The negatively polarized oxygen acts as a hydrogen bond Figure 1: Formation of a hydrogen bond
acceptor, while the hydrogen attached to the oxygen serves as a hydrogen between ethanol molecules (n.d.)
bond donor, forming a hydrogen bond. As there are both hydrogen bond acceptor and donor in the same molecule, a
strong network of alcohol molecules is created in the liquid phase that requires more energy to break apart. This is
why the boiling points of alcohols are significantly higher than that of alkanes of comparable molecular mass.
In Chemistry lessons we also learned that increasing the carbon chain length of organic compounds increases its
overall London dispersion forces, which increases the compound’s boiling point. Following this, I wondered what
factors could affect the other intermolecular forces found between molecules. Alcohols are the perfect homologous
series to investigate in this regard as they exhibit all 3 IMFs; namely London dispersion forces, dipole-dipole forces
and hydrogen bonding. Isomerism is the phenomenon in which different compounds have the same molecular
formula but differ in their chemical structures. In this investigation, I will be looking at 3 types of structural
isomerism; positional isomerism, chain isomerism and metamerism.
Positional isomerism
Positional isomerism is a type of structural isomerism in which each isomer differs in terms of position of the
functional group on the carbon skeleton. In alcohols, the position of the hydroxyl group affects how exposed the
surface of the oxygen atom is for hydrogen bonding with other alcohol molecules. Theoretically, as the functional
group moves inwards on the carbon chain it becomes more shielded by the surrounding alkyl groups, which makes
, 2
the oxygen atom less accessible to hydrogen atoms in neighbouring alcohol molecules, thus reducing the likelihood
of forming a strong hydrogen bond. Hence isomers with the hydroxyl functional group more exposed and on the
outside of the carbon chain should be having higher boiling points than isomers with the functional group less
exposed and on the inside of the carbon chain.
Chain isomerism
Chain isomerism occurs due to branching of carbon chains on the main carbon skeleton. A straight chain organic
compound has a higher surface area that allows for more Van der Waal interactions to take place between
molecules, increasing the total strength of the IMFs and hence raising the boiling point. On the other hand,
branching makes the molecule more compact, reducing the surface area available for Van der Waal interactions, and
contributes to a lower boiling point.
Figure 2: Van der Waals forces in an aliphatic vs. branched hydrocarbon
The effect of chain isomerism can be measured through solvent accessible surface area, as surface area is
proportional to the Van der Waals forces of the molecule.
Metamerism
Ethers are isomers of alcohols with an ether functional group instead of hydroxyl, that consists of an oxygen atom
forming single bonds with two alkyl groups and have the formula R-O-R'. Similar to alcohols, these compounds find
use in production of dyes, perfumes, oils, waxes and other industries. The strongest intermolecular forces in ethers
are dipole-dipole forces instead of the hydrogen bonds that are present in alcohols. As a result of this ethers possess
significantly lower boiling points than alcohols of similar molecular mass. Due to high electronegativity of the
oxygen atom along with the lone pair of electrons it possesses, the shared pair of electrons between the alkyl groups
and the ether functional group is pulled more tightly towards the oxygen atom, forming a dipole moment (“Dipole
Moments”). Because of the lone pair of electrons on the oxygen atom, the overall molecular geometry of ether
molecules are bent (via VSEPR theory) which means that the vectors representing the dipole moment do not cancel
each other out, giving the molecule a net dipole moment.
Figure 3: Dipole moment in an ether molecule (“Names and Properties of Ethers”)
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