On Alcohols – Some High School level Chemistry

Alcohols are organic chemical compounds that consist of a hydroxyl group (-OH) attached to one or more carbon atoms within an alkane structure. Alcohols are a homologous series and have the general formula of CnH2n+1OH.

Examples of common alcohols include:

 

The OH attached within the alcohols result in higher melting and boiling points than expected for a compound of similar molecular mass. The hydroxyl group is a form of hydrogen bonding which is the strongest intermolecular force and gives rise to their stronger structure. This strong molecular structure takes more energy to break than the ones in compounds that are held together by London Dispersion Forces (weakest intermolecular force) or Permanent Dipole-Permanent Dipole attractions.

For example:

 

difference-between-methane-and-propane-2
Propane – mass=44g b.p.t=-42 deg C
3-ethanol
Ethanol – mass=46g b.p.t=78 deg C

There are three types of alcohols; Primary, Secondary and Tertiary. Alcohols are thereby classified as either one of these three types, depending on the position of the hydroxyl group in the carbon chain. Each of the three types have some similar and distinct chemical properties, which also enables us to distinguish between them.

In a primary alcohol, the OH group is attached to the carbon atom at the end of the chain which also has two additional hydrogen atoms bonded to it.

E.g.

propanol_flat_structure
Propan-1-ol

 

In a secondary alcohol, the OH group is attached to a carbon atom within the chain which has one additional hydrogen atom bonded opposite it.

E.g.

99d226c43547f1934fbc5d4e8a52a4d6
Pentan-2-ol

 

In a tertiary alcohol, the OH group is also attached to a carbon atom within the chain, however, this time it has a branch group bonded opposite instead of a hydrogen atom.

E.g.

2-methylbutan-2-ol
2-methylbutan-2-ol

These alcohols can be distinguished through oxidation with a chemical such as potassium dichromate. The success of alcohol oxidation causes a reduction of dichromate ions which turns the solution from its initial orange colour to green. Both primary and secondary alcohols can be oxidised further whereas tertiary alcohols do not as there are no observable changes in the colour of the solution.

Primary alcohols oxidise to compounds named Aldehydes and secondary to Ketones which both contain the carbonyl functional group. Aldehydes and Ketones differ due to the difference in the positions of the carbonyl group. The functional group appears at the end of the carbon chain for Aldehydes and within the chain for Ketones.

Structures:

download

The C=O group is created as the O:H ratio increases through the loss of hydrogen atom from the OH group itself and its bonded carbon. Since tertiary alcohols do not have the additional hydrogen but rather a branch group opposite suggests that they cannot undergo oxidation.

P.s – update post #2 on Dark matter may be next

Author – Susan Chen

Susan is a 5th year high school student currently studying three STEM subjects at Scottish Higher level-Mathematics, Physics and Chemistry (Crash Course). She particularly loves ideas in cosmology and hopes to embark on an academic journey in the area of theoretical physics.

Advertisements

6 thoughts on “On Alcohols – Some High School level Chemistry

  1. Jiangmin Hou February 13, 2017 / 3:57 pm

    你的博客完美地总结了我们的化学课程!坡坡会欣慰😊

    Liked by 1 person

      • Jiangmin Hou February 13, 2017 / 7:47 pm

        不用谢,老朋友/神童!

        Liked by 1 person

      • Jiangmin Hou February 13, 2017 / 7:50 pm

        连点赞也神速

        Liked by 1 person

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s