53.TYPES OF ORGANIC FORMULAE -(3) – Structural Formulae.

In this post we shall study how to draw structural formulae for different organic molecules. Understanding the structural representation of molecules is very important in our later course of study. We shall study in detail the stereochemistry( i.e spatial arrangement) of molecules in upcoming posts , and one can only grasp the topic if the structural representation can be correctly visualized. Hence, this post can be regarded as a basis to understanding structures of  various molecules and their stereochemistry.

To emphasize the importance of understanding structures , let us consider a very simple molecule , Methane.

The formula for methane is CH4. However, this does not fully tell us about the molecule. The following formula helps us a bit better in understanding the structure – 531

Yet it does not tell us how the atoms are arranged in space. The following structure helps us with that information – 532

This was a very simple example, but as the structures start getting complicated, it becomes very important to correctly represent them, as a little change in the spatial arrangement of an atom could change the properties of the molecule totally!

Let us begin studying how molecules are represented in chemistry.


Lewis Structures

 

Gilbert_N_LewisGilbert Newton Lewis was a member of the Royal society and the chemist who introduced the concept of covalent bonds and Lewis/dot structure  to represent covalent bonds.

Lewis structures –

♦Only valence electrons are considered while drawing Lewis structures.Valence electrons are represented by dots(•) ,thus the name dot structures.The advantage of drawing these structures is that they exactly tell us how many shared paired of valence electrons and how many lone pair of electrons exist in the molecule.Knowledge of this arrangement helps us in understanding the geometry of molecules. We shall study more about this when we discuss VSEPR theory.

♦Begin by counting total number of valence electrons. Use the periodic table and electronic configurations of atoms to determine how many valence electrons each atom has.Then simply add the total valence electrons.

no of valence electrons in a compound = ∑ (no of valence electrons on individual atoms).

(NOTE – Generally,
 # of valence electrons= Group number in periodic table.
Carbon is in group 4 so it has 4 valence electrons).

e.g.- Consider , Acetaldehyde (CH3CHO).
# valence electrons in 
CH3CHO = 2(# valence electrons in carbon)+ 4(# valence electrons in Hydrogen)+ #(valence electrons in oxygen).
= 2(4) +4(1) + 6
= 18 electrons.

♦Place the least electronegative atom in the centre(except hydrogen for organic compounds).We have already studied the periodic trends in electronegativity in post # 46. We know that ,

F > O > Cl > N > Br > I > S > C > H > metals

463

So, for a molecule like NO2, nitrogen will be in the centre flanked by two oxygen atoms.
O-N-O (a rough figure).

♦Then, start placing the electrons in pairs on the more electronegative atoms first until their octet is complete.

 

 

538
Lewis structure for acetaldehyde.

As seen in the above example, we first we complete oxygen atom’s octet. So, out of total 18 electrons, 8 are arranged.Then we start completing two carbon atom’s octet with the remaining 10 electrons.Thus, we arrive at acetaldehyde’s lewis / dot structure. (The final structure has 18 electrons and octets of Carbon and oxygen are complete. The duplet of hydrogen is complete too).The electron pairs on oxygen ,which are not shared with any other atom are LONE PAIRS of electrons.
4 electrons are shared between carbon and oxygen indicating that there is a DOUBLE BOND between them.

 

539.jpg

 Condensed Formula

This form of notation , uses text form only to represent molecules.
e.g. – Methane =CH4
Propane = CH3CH2CH3.

Parentheses/Brackets are used to indicate multiple identical groups.These brackets simplify a long formula.
e.g. –
♦ Decane. Instead of writing CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3, it is more convenient to write CH3(CH2)8CH3.
♦ Carbonyl oxygen atoms (C=O) can also be written in brackets.
e.g. 
CH3C(O)CHimplies that the oxygen atom is attached to the second carbon atom.
♦ Branching in hydrocarbon chains can also be depicted using brackets.
e.g.- 
CH3CH(CH3)CH2CH3. This formula indicates that the CH3 group in the bracket is on second carbon atom i.e there is branching at C-2 atom.533

Rule : look to the left of the bracket to see which atom it’s attached to. 

534.jpg

The condensed formula can be written for aliphatic compounds but for complicated structures and cyclic structures these formulae are unusable.


Skeletal formula

These formulae are like shortcut structures to the dash structures of organic molecules. This formula is useful in writing complex molecules. It is very important that one practises writing these as they can get a bit confusing.

Frkekulé.jpg

The concept of skeletal structures was introduced by German Chemist , Kekulé – the man credited to have found out the Benzene structure!

In this case, one does not draw the following –

1.Carbon atoms – they are assumed to be at the vertices of the line segments.
540.jpg
2.Hydrogen atoms – Whenever tetravalency of a  carbon atom is not fulfilled, it is supposed to have hydrogen/s , which will fulfill all the valencies of carbon atoms.
5391.jpg

3.Lone pairs – Lone pairs are not shown. We are expected to know where they are present in the molecule.

5393

4.Formal charges – Formal charge on species is not shown, if there is an ionic end to a organic molecule.
5392

5394


 

There is a lot more to structures in organic chemistry .Depicting the correct spatial arrangements of atoms is a whole new chapter called stereochemistry. We shall study the organic structures in much greater detail in that chapter.

In this post , we have just discussed the basic structural representation of molecules , used commonly in organic chemistry.Hope you got a gist of how molecules are represented in chemistry. We shall continue our posts on organic chemistry.Till then,

Be a perpetual student of life and keep learning..

Good day !

 

References and further reading –
1.http://www.masterorganicchemistry.com/2011/06/20/deciphering-what-the-brackets-mean/
2.https://en.wikipedia.org/wiki/Structural_formula

Image source –
1.https://www.alphachisigma.org/page.aspx?pid=486
2.By Unknown – http://ihm.nlm.nih.gov/images/B16099 Uploaded by en:User:Maximus Rex., Public Domain, https://commons.wikimedia.org/w/index.php?curid=167877

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