Reactions of organic compounds
Addition reactions
The most typical reactions of alkenes and alkynes are the addition reactions, in which reactant adds to the carbon atoms of the C=C double or C≡C triple bond.Alkenes
In the case of alkenes the p bond of the double bond is broken and two new groups are attached to give a saturated compound. Reagents such as the halogens (Cl2, Br2, and I2), the hydrogen halides (HCl and HBr), and water commonly add to the double bonds of alkenes to give saturated compounds.
Addition of HCl to ethene Bromination of cyclopenthene
The catalytic hydration of alkenes is a method for preparation of alkanes. In fact this is a process of addition of H2 to the double p bond.
Catalytic hydration of ethylene
Addition of unsymmetrical substance like HX to an unsymmetrical alkene can theoretically give two products.
The main rule for unsymmetrical addition reactions is Markovnikov's rule: “In addition of HX to an unsymmetrical carbon-carbon double bond, the hydrogen of HX goes to that carbon of the double bond that carries the greater number of hydrogens. Thus according to Markovnikov's rule:
Hydrogen chloride will add to propylene (propene) giving 2-chloropropane:
Hydrogen chloride will add to isobutylene (2-methyl-1-propene) giving 2-chloro-2-methylpropane:
Alkynes behave as typical unsaturated compounds with two double linkages. Bromine adds to acetylene (ethyne) in two steps: first to give 1,2-dibromoetylene, and finally, to produce 1,1,2,2-tetrabromoethane.
1,2-dibromoethylene 1,1,2,2-tetrabromoethane
The product of addition of one molecule of water to acetylene is unstable and rearranges to acetaldehyde, a carbonyl compound.
vinyl alcohol acetaldehyde(unstable)
Addition reactions for unsymmetrical alkynes proceed in accord with Markovnikov's rule.
Unsymmetrical alkynes undergo addition of hydrogen halides according to Markovnikov's rule in both steps of the reaction.
propyne 2-bromopropene 2,2-dibromopropane
With alkyl-substituted acetylenes, addition of water always occurs in accord with Markovnikov's rule.
The addition of H2 to carbonyl group of
aldehydes primary
and results in formation of and alcohols.
ketones secondary
The reaction known also as reduction of carbonyl compounds is the easiest method for conversion of aldehydes and ketones to alcohols.
The addition of H2O to carbonyl group leads to formation of diols.
methanal methanediol
Hydrogen cyanide adds to many aldehydes and ketones to give cyanoalcohols, usually called cyanohydrins.
acetone 2-hydroxy-2-cyanopropane
Addition of HCN to a carbonyl group
What is electrophilic addition? http://www.chemguide.co.uk/mechanisms/eladd/whatis.html#top
Simple mechanism of the electrophilic addition reactions between the hydrogen halides and alkenes like ethene and cyclohexene. http://www.chemguide.co.uk/mechanisms/eladdmenu.html#top
The mechanism of the reaction between ethene (and cyclohexene) and bromine. http://www.chemguide.co.uk/mechanisms/eladd/symbr2.html#top
Markovnikov's rule: definition and discussion. http://www.chemguide.co.uk/mechanisms/eladd/unsymprob.html#top
Mechanism of the electrophilic addition reactions between hydrogen halides and antisymmetrical alkenes. http://www.chemguide.co.uk/mechanisms/eladd/unsymhbr.html#top
Simulation for the Markovnikov addition of HCl to propene.
Elimination reactions can be considered as reverse process of the addition to the double bond in alkenes or triple bond in alkynes. In general, an alkyl derivative will, under appropriate conditions, eliminate HX, where X is commonly a halide or a hydroxyl, and the hydrogen is located on the carbon adjacent to that bearing the X function.
Dehydration of alcohols
When an alcohol with H and OH on adjacent carbon atom undergoes elimination reaction, the alcohol is dehydrated to give the corresponding alkene. This process is just the reverse of the reaction used to make ethanol from ethylene. The process of dehydration is taking place in presence of concentrated sulphuric acid.
ethanal ethylene
Elimination of hydrogen halide from halogenoalkanes
Halogenoalkanes can undergo elimination reaction in the presence of hydroxide ion (from KOH or NaOH) to give the respective alkene. For example, when 2‑bromopropane is heated under reflux with a concentrated solution of sodium or potassium hydroxide in ethanol, propene is formed.
2-bromopropane propene
Elimination of HBr from ethylbromide
Elimination of HBr from tret-buthylbromide
Internet resources:
Mechanism of elimination reaction See animation http://neon.cm.utexas.edu/academic/resources_movies/movies/iverson/main.htm
Elimination reaction between a simple halogenoalkane like 2-bromopropane. http://www.chemguide.co.uk/mechanisms/elim/elim.html#top
This page looks at elimination from unsymmetric halogenoalkanes such as 2-bromobutane. http://www.chemguide.co.uk/mechanisms/elim/elimunsym.html#top
This page gives you the facts and a simple, uncluttered mechanism for the acid catalysed dehydration of a simple alcohol like ethanol to give an alkene like ethene. If you want the mechanism explained to you in detail, there is a link at the bottom of the page. http://www.chemguide.co.uk/mechanisms/elim/dhethanol.html#top
Dehydration of more complicated alcohols. http://www.chemguide.co.uk/mechanisms/elim/dhcomplex.html#top
Elimination versus substitution in halogenoalkanes. This page discusses the factors that decide whether halogenoalkanes undergo elimination reactions or nucleophilic substitution when they react with hydroxide ions from, say, sodium hydroxide or potassium hydroxide. http://www.chemguide.co.uk/mechanisms/elim/elimvsubst.html#top
In general reactions undergoing through substitution of one (or more atoms) in a molecule for another atom are called substitution reactions. Examples for different type of organic substitution reactions follow:
Radical substitution reactions – replacement of one or more H atoms on an alkane by some other atom – a halogen. This type of reactions undergo in the presence of ultraviolet radiation or at high temperatures. Free radicals can be obtained at those conditions. For example, complete reaction of methane with chlorine in the presence of ultraviolet radiation or at high temperature yields CCl4, commonly known as carbon tetrachloride.
These reactions can be considered to involve three phases. First, chain initiation must occur, which for methane chlorination is activation and conversion of chlorine molecules to chlorine atoms by light.
chain initiation
In the second phase, the chain propagation steps converts reactants to products: 1) the chlorine atom can remove a hydrogen atom from a methane molecule and form a methyl radical and hydrogen chloride molecule; 2) the methyl radical can then remove a chlorine atom from molecular chlorine and form methyl chloride and a new chlorine atom.
chain propagation
The propagation reactions occur in competition with chain terminating steps, where chlorine atoms or methyl radicals are destroyed by reacting with one another, as shown in the following equations:
This type of process is called chain reaction since, in principal, one chlorine atom can induce the chlorination of an infinite number of methane molecules through operation of a “chain” or cycle of reactions.
Internet resources:
What is free radical substitution? http://www.chemguide.co.uk/mechanisms/freerad/whatis.html#top
Free radical substitution of hydrogen atoms in methane by bromine atoms. http://www.chemguide.co.uk/mechanisms/freerad/ch4andbr2.html#top
Free radical substitution of hydrogen atoms in the methyl group in methylbenzene by chlorine atoms. http://www.chemguide.co.uk/mechanisms/freerad/tolueneandcl2.html#top
Benzene and other aromatic compounds undergo electrophilic substitution reactions – substitution of one or more H atoms of benzene for a halogen atom, a nitro group, or an alkyl or other hydrocarbon grouping.
It is important to realise that in aromatic substitution the electrophilic substituting agent is not necessarily the reagent that is initially added to the reaction mixture.
Nitration process is realised by the attack of nitronium ion, NO2+, that is formed from nitric acid and sulphuric acid according to the following equation:
The mechanism of halogenation is completed by the fact that molecular halogens, Cl2, Br2 and I2, form complexes with aromatic hydrocarbon. A catalyst is usually necessary, and the catalysts most frequently used are metal halides (FeBr3, AlCl3, and ZnCl2).
Alkylation is an important method of synthesis of alkyl benzenes. The method utilizes an alkyl halide as the alkylating agent together with a metal halide catalyst, usually aluminium chloride AlCl3. This class of reactions is familiarly known as Friedel-Crafts alkylation.
Internet resources:
What is electrophilic substitution? http://www.chemguide.co.uk/mechanisms/elsub/whatis.html#top
Animation of the nitration of benzene 
See animation http://neon.cm.utexas.edu/academic/resources_movies/movies/iverson/main.htm
http://www.mp-docker.demon.co.uk/chains_and_rings/mechanisms/elec_sub.html
Different animations http://www.mp-docker.demon.co.uk/chains_and_rings/mechanisms/index.html
Nucleophilic substitution reactions
Broadly defined, a substitution reaction involves the replacement of one functional group (X) by another (Y):
When the reaction involves the attack by a nucleophile (an electron donating reagent) at carbon atom, it is called nucleophilic substitution. The nucleophile may be an anion (Cl¯, Br¯, OH¯ ) or a neutral molecule (NH3, H2O, CH3OH). A typical example is the reaction of hydroxide ion with methyl halide to displace halogen ion.
Substitution of Br with hydroxide group resulting an alcohol
More complex example is the nucleophilic acyl substitution, which can be described as an addition/elimination process.
CH3COCl + NH3 ® CH3CONH2 +HCl
First step is the addition of ammonia molecule to carbon atom from the carbonyl double bond, followed by the second step – elimination of HCl molecule and creation of amide as a product of the reaction.
Nucleophilic acyl substitution
Internet resources:
What is nucleophilic substitution? http://www.chemguide.co.uk/mechanisms/nucsub/whatis.html#top
Mechanism of the nucleophilic substitution See animation http://neon.cm.utexas.edu/academic/resources_movies/movies/iverson/main.htm
The nucleophilic substitution reactions between halogenoalkanes and hydroxide ions http://www.chemguide.co.uk/mechanisms/nucsub/hydroxide.html#top
The nucleophilic substitution reactions between halogenoalkanes and hydroxide ions http://www.chemguide.co.uk/mechanisms/nucsub/hydroxide.html#top
Substitution reactions involving water http://www.chemguide.co.uk/mechanisms/nucsub/water.html#top
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