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Introduction to Organic Chemistry

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Hydrocarbons

Unsaturated Hydrocarbons

Alycyclic Hydrocarbons

Aromatic Hydrocarbons

Functional Groups

Reactions of Organic Compounds

Types Of Reactions

Types Of Bond Breaking

Types Of Reagent

Reactions Of Alkanes

Alcohols and Ethers

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Hydrocarbons

Compounds of carbon and hydrogen are called hydrocarbons.  The simplest ones are called alkanes, in which all the C - C bonds are single bonds.  They are said to be saturated because of this.  The general formula of alkanes is C_nH_2n+2.  A series of compounds with similar structure and chemical properties, in which members differ from one another by the possession of an additional CH₂ group, is called a homologous series.  The first ten members of the alkane series are:

The basis of naming organic compounds is that the name consists essentially of a Stem + Suffix.  The stem gives the number of carbon atoms in the longest straight chain.  The suffix gives the principal group, called functional group, or groups in the compound.  Saturated hydrocarbons do not have a functional group and the suffix used for these compounds is -ane.

It is possible to have alkanes with straight or branched chains, e.g. it is possible for, C₄H₁₀, to have the atoms arranged in two different ways.  These are called isomers.

Branched chains are named by regarding them as consisting of a straight chain with one or more side chains attached.  There is a procedure to follow:

1. Find the longest main carbon chain and name it.
2. Number the carbon atoms in the main chain starting at one end.
3. Look for the side groups and identify the number of the carbon atom(s) to which they are attached.

If there are more than one side groups, then separate numbers must appear in the name for each of them:

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Unsaturated Hydrocarbons

The alkanes are not the only hydrocarbons.  There are also alkenes and alkynes.  They are unsaturated hydrocarbons; they contain multiple bonds between carbon atoms.  The simplest alkene is ethene, H₂C=CH₂.  It is the first member of the homologous series of alkenes, which have the general formula C_nH_2n.  Naming is similar to the alkanes but the suffix is now -ene instead of -ane. There is usually a number indicating the position of the double bond.

Alkynes contain one or more carbon-carbon triple bonds.  The simplest is ethyne, HC≡CH.  It is the first member of the homologous series of alkynes, which have the general formula, C_nH_2n-2.

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Alicyclic Hydrocarbons

Another set of hydrocarbons are the alicyclic hydrocarbons.  These contain rings of carbon atoms.

Examples are:

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Aromatic Hydrocarbons

These are related to benzene. Members of this group, the arenes, are benzene, C₆H₆; methyl benzene, C₆H₅CH₃; naphthalene, C₁₀H₈.

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Functional Groups

The double bond in the alkenes is responsible for most of the chemical reactions of these compounds; in other words they contain the functional group  .  Similarly the alkynes contain the functional group  .

The halogenoalkanes have the formula RX, where R is the alkyl group and X is a halogen.  An alkyl group has the general formula C_nH_2n+1.

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Reactions of Organic Compounds

Types of Reactions

1. Substitution.  An atom or group of atoms replaces another:
   
   
   CH₃Br + OH^- CH₃OH + Br^-
2. Addition.  Two molecules react to form one:
   
   
   CH₂=CH₂ + Br₂ CH₂BrCH₂Br
3. Elimination.  One molecule reacts to form more than one:
   
   
   C₂H₅OH C₂H₄ + H₂O

Types of Bond Breaking

When a bond breaks, each of the bonded atoms takes one of the pair of electrons.  Radicals are formed.  These are atoms or groups of atoms with unpaired electrons.

1. Homolytic Fission.  When the bond breaks, each of the bonded atoms takes one of the electrons; radicals are formed.
   
   
   Cl - Cl Cl · + · Cl
2. Heterolytic Fission.  When the bond breaks, the electrons are unequally distributed between the two atoms, forming ions.
   
   
   H - Cl H⁺ + Cl^-

Types of Reagent

1. Nucleophiles are attacking species with a pair of electrons available for forming a new covalent bond.  They are often negatively charged.  They attack the positive end of a polar bond.  E.g.    OH^-,   NH₃, CN^-.
2. Electrophiles are attacking species with a vacancy for a pair of electrons.  A new covalent bond results when the vacancy is filled.  Electrophiles are often positively charged.  They attack regions of high electron density.  E.g.    H⁺,   NO₂⁺,   SO₃.

Reactions of Alkanes

Alkanes have two important uses: as fuels and as the foundation of the petrochemicals industry.

1. Alkanes readily burn in oxygen to produce carbon dioxide and water.
   
         CH₄ + 2O₂ CO₂ + 2H₂O
2. High molecular mass Alkanes can be converted into smaller molecules by the action of heat and an alumina catalyst.  A mixture of products occurs, one of which is an alkene.  The process is called cracking.
   
         E.g. C₁₁H₂₄ C₉H₂₀ + C₂H₄
3. Alkanes react with halogens in the presence of sunlight, when substitution occurs
   
         CH₄ + Cl₂ CH₃Cl + HCl

The reaction can continue until all the hydrogen atoms are substituted by chlorine atoms, yielding eventually, CCl₄.

Much work has been done on how this reaction occurs, when methane is so unreactive towards other reagents.  The reaction occurs rapidly in sunlight or above 300°C.  It is an example of a photochemical reaction.  The reaction proceeds by a number of steps involving radicals.

1. Chain Initiation
         Cl₂ hν + 2Cl·
   
   The necessary energy comes from the light or heat.
   
   
2. Chain Propagation
         Cl · + CH₄ HCl + ·CH₃
         ·CH₃ + Cl₂ CH₃Cl + ·Cl
   
   
3. Chain Termination
       Radicals are very reaction and are able to react with each other and terminate the reaction.
         2 ·Cl Cl₂
         2 ·CH₃ C₂H₆
         ·Cl + ·CH₃ CH₃Cl

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Alcohols and Ethers

There are two isomers with the molecular formula C₂H₆O:  namely ethanol, CH₃CH₂OH and methoxymethane, CH₃OCH₃.

Ethanol is an alcohol.  All alcohols have the hydroxyl functional group, -OH, and exhibit similar chemical properties.  Their general formula is R-OH.

Methoxymethane is an ether.  All ethers contain the alkoxy functional group -OR and they shew similar chemical properties.  Their general formula is R-O-R'.

Alcohols are derived from the alkanes by substituting an -OH group for a hydrogen atom.

For alcohols containing more than two carbon atoms, isomers are possible.  To distinguish between them, it is necessary to label the position of the OH group.  The hydrocarbon-chain is always numbered from the end which gives the lowest number for the position of the functional group.  Some alcohols have more than one -OH group.  These are called diols: for example CH₂OHCH₂OH is called ethane-1,2-diol.

Like water molecules, alcohols are polar due to the elctronegativity of the oxygen atom.  Alcohols are therefore able to hydrogen bond to each other and to water molecules.  This explains their solubility in water.

Ethers can be thought as being derived from water with both hydrogen atoms being replaced by alkyl groups.  As there are no hydrogen atoms attached to the oxygen, so ethers cannot form hydrogen bonds between molecules.  Ethers are only slightly soluble in water.

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