Introduction to Organic Compounds

Introduction

The  basis  of  Organic  Chemistry  is  the  fact  that  Carbon  molecules  can  produce  long  chains  which  can produce a number of different molecules. The reason why Carbon is such an ideal building block is due to following reasons:

     Carbon-Carbon bonds are very strong.

     Carbon can bond with 4 different other atoms.

     Can produce single, double and triple bonds.

     Has a very similar electronegativity to Hydrogen, making a hydrocarbon chain highly       unreactive.

Drawing an organic compound

There are several ways to draw an organic compound, mainly being display formulae, 3D structure and skeletal structure.

Display formulae

This is a picture of the compound showing all of the bonds present in the compound. An example is ethane:

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3D structure

Simple organic chains can be drawn as a 3D structure, by using the following convention:

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which can thus show the orientation of the molecule. Ethane in 3D would be shown as follows:

introducing Organic Compounds 3.jpg

Structural formula

A structural formula is normally used for long chained chains, in which only the Carbons are drawn and any functional groups that are attached to the chain. Butan-2-ol would be drawn as follows:

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Naming of alkanes

When naming a carbon chain it is of utter importance to note the longest Carbon chain. This would be the basis of the naming and the prefixes are as follow:

Longest Carbon Chain           Prefix

1                                                Meth-

2                                                Eth-

3                                                Prop-

4                                                But-

5                                                Pent-

6                                                Hex-

7                                                Hept-

8                                                Oct-

9                                                Non-

10                                              Dec-

Rules for Naming Alkanes

Simple Alkanes

The parent name of the molecule is determined by the number of carbons in the longest chain.

In the case where two chains have the same number of carbons, the parent is the chain with the most substituents.

The carbons in the chain are numbered starting from the end nearest the first substituent.

In the case where there are substituents having the same number of carbons from both ends, numbering starts from the end nearest the next substituent.

When more than one of a given substituent is present, a prefix is applied to indicate the number of substituents. Use di- for two, tri- for three, tetra- for four, etc. and use the number assigned to the carbon to indicate the position of each substituent.

Cyclic Alkanes

The parent name is determined by the number of carbons in the largest ring (e.g., cycloalkane such as cyclohexane).

In  the  case  where  the  ring  is  attached  to  a  chain  containing  additional  carbons,  the  ring  is considered  to  be  a  substituent  on  the  chain.  A  substituted  ring  that  is  a  substituent  on something else is named using the rules for branched alkanes.

When two  rings are attached to  each other, the larger  ring is the  parent  and  the smaller  is a cycloalkyl substituent.

The carbons of the ring are numbered such that the substituents are given the lowest possible numbers.

Isomerism

Isomerism  is  a  phenomenon  were  different  molecules  having  the  same  molecular  formula  can  be produced,  with  the  most  important  isomerism  being  functional  isomerism.  This  would  mean  that  the molecule would have the same number of atoms, but these would be positioned in a different order, for example:

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where the two molecules have got the same number of Carbon and Hydrogen atoms but the way the

Carbons are attached to each other is in a different order. This is called Chain isomerism.

A different type of functional isomerism  is when another functional group is present, which might be positioned on a different Carbon. An example would be:

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where the Bromine is found on a different Carbon. This is called position isomerism.

Branched Alkanes

Branched  substituents  are  numbered  starting  from  the  carbon  of  the  substituent  attached  to the  parent  chain.  From  this  carbon,  count  the  number  of  carbons  in  the  longest  chain  of  the substituent. The substituent is named as an alkyl group based on the number of carbons in this chain.

Numbering of the substituent chain starts from the carbon attached to the parent chain.

The entire name of the branched substituent is placed in parentheses, preceded by a number indicating which parent-chain carbon it joins.

Substituents  are  listed  in alphabetical order. To  alphabetize,  ignore  numerical  (di-,  tri-, tetra-) prefixes  (e.g.,  ethyl  would  come  before  dimethyl),  but  don’t  ignore  don’t  ignore  positional prefixes such as iso and tert (e.g., triethyl comes before tertbutyl).

Empirical Formulae

An empirical formula is a formula in which the minimal ratio of all the constituents of each compound is written  in  the  formula.  In  order  to  change  an  empirical  formula  to  a  molecular  formula  the  ratio between the molecular weight and the empirical weight would be found and then the empirical formula would be multiplied with this ratio.

  1. Start with the number of grams of each element, given in the problem.

     If  percentages  are  given,  assume  that  the  total  mass  is  100  grams  so  that

the mass of each element = the percent given.

  1. Convert the mass of each element to moles using the molar mass from the periodic table.
  2. Divide each mole value by the smallest number of moles calculated.
  3. Round to  the  nearest  whole  number.   This  is  the  mole  ratio  of  the  elements  and  is represented by subscripts in the empirical formula.

If the number is too far to round (x.1 ~ x.9), then multiply each solution by the same factor to get the lowest whole number multiple.

e.g.  If one solution is 1.5, then multiply each solution in the problem by 2 to get 3.

e.g.  If one solution is 1.25, then multiply each solution in the problem by 4 to get 5.

Hybridisation

Hybridisation is the concept of mixing orbitals to form new hybrid orbitals which would allow different orientations to take place in order to be able to produce the greatest possible angles between different bonds.

An  atom  that  can  be  used  to  show  different  types  of  hybridisation  is  C,  since  this  can  produce  three different hybrid orbitals, mainly sp3, sp2  and sp.

The ground state of C is:                                                             The excited state of C is:

introducing Organic Compounds 7                          introducing Organic Compounds 8

In an sp3  hybridisation the four orbitals would then recombine to form 4 new hybrid orbitals, which can then rearrange to form a tetrahedron. this can be seen as follows:

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In an sp2  only three orbitals are hybridised while the last p orbital would remain unchanged, producing a planar compound with a perpendicular orbital. This can be seen as follows:

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In an sp hybridised orbital the hybridisation occurs only between an s orbital and one of the 3 p orbitals producing two sp hybrid orbitals and leaving two p orbitals. This can be seen as follows:

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