Question

a) describe a branched hydrocarbon
b) what are the steps for naming a branched hydrocarbon
c)how does the branching of a hydrocarbon affect the physical and chemical properties? why?
d)what are saturated and unsaturated hydrocarbons? which hydrocarbons are also known as saturated hydrocarbons?

Answer 1

a) A branched hydrocarbon is a hydrocarbon where at least one of its C atom is connected by a direct covalent bond to atleast 3 other C atom in the molecule.

For example, In isobutane, one of the carbon is connected to 3 other carbon. Hence it is a branched hydrocarbon.

This carbon is connected to 3 other C.Hence branched.

We can compare this to an unbranched hydrocarbon like, n-pentane.

$CH_3-CH_2-CH_2-CH_2-CH_3$

Here all the carbon atoms are connected to either one or two other carbon atom, hence unbranched.

B)

• First step of naming a branched hydrocarbon is to find out the longest possible straight chain of C-C in the molecule. For example: in the above structure of isobutane the longest straight chain possible consists of 3 carbon atoms.
• Second step is to find out all other substituents attached to the straight chain you found out on step one. If two substitutents are same you have to use di-, tri-, tetra- infront of them depending on their number.
• Step three is to number the straight chain in such a way that the substituents are attached to the carbons with smallest number.
• Now, right the name of the substituents alphabetically(without considering the di-, tri- part) with prefix of the carbon numebr they are attached to. For example, in isobutane the methyl group is attached to carbon 2, Hence, it will be written as 2-methylpropane.
• The last part is naming the straight chain alkane, in case of isobutane the straight chain is 3 carbon long, hence the suffix propane.

Here is 2,3 - dimethylpentane as another example.

CH3 Longest straight chain of 5 Carbons 5 3 1 4 HaC CH 3 снз two methyl substituents

c) The unbranched hydrocarbons are linear. Hence they are easier to line up against each other resulting in a more compact structure. Hence more energy is need to break their van der waals attraction. This results in higher boiling point of unbranched alkanes.

However in case of branched hydrocarbons have smaller area of contact between the molecules resulting in weak van der waals force between them, hence lower boiling point.

Chemically, they differ a lot as to how they form products in different kind of reaction based on their stability of cations and anions formed. It is because the stabilizing effect such as inductive effect differ in branched and unbranched hydrocarbon when they form cation, anions or radicals, the active species that undergoes reaction.

d) Saturated hydrocarbons are those, which does not contain any pi- bond in them. In other words there is no C=C or $C\equiv C$ present in the saturated hydrocarbons. The "saturation" term here means saturated with hydrogen. In saturated hydrocarbon, the carbon atoms are bonded to maximum possible number of hydrogen. For example, Propane

$CH_3-CH_2-CH_3$. It has only single bonds.

However, in unsaturated hydrocarbons, there is atleast a C=C present. It is called unsaturated because it still can accept a H2 molecule per pi bond to make it saturated.

For example, ethylene:

$CH_2=CH_2$. Here there is a double bond between two C atoms.

Alkanes are otherwise known as saturated hydrocarbons. It is because they do not contain any pi bond in them.

For example, methane, ethane, butane, propane, isobutane, 2,3-dimethylpentane, all of these are examples of saturated hydrocarbons.