Mechanisms 1

Here are the mechanisms for 3 types of reactions. Free Radical Substitution for alkanes, Electrophilic Substitution for alkenes and Nucleophilic Substitution for Halogenoalkanes/Alkyl Halides.

Alkanes mainly go through Free Radical Substitution (FRS) reactions, combustion as well as cracking. Since i have not included any mindmaps on alkanes, i shall proceed to describe more about alkanes for our syllabus. Hydrogen atoms are substituted by halogen atoms in FRS.

Combustion reactions for alkanes have a general equation of:

C_xH_y + ( x + y/4) O₂ (g)→ xCO₂ (g) + (y/2) H₂O (l)

Typical alkane questions would usually involve the application of this equation so i think it's important to memorise it. There will be clues to helping you find the values of x and y in the equation. One particular type i noticed is that they like to do combustion then run the resultant gas through limewater to remove the CO_{2. From there, just determine the number of moles of CO2} and you have found the value of x! Note that water formed is in liquid state, not gaseous state in the equation.

Another reaction to remember that alkanes undergo is cracking, which involves the breaking up of a large molecule to smaller molecules. Hydrogen gas is produced from cracking reactions.

For more information about Halogenoalkanes or Alkenes, please refer to the Chemistry section and search for a post with the appropriate title.

Mechanisms for the 3 reactions:

Iodoform test

The iodoform test (or tri Iodomethane test) is used to identify the presence of 2 particular structural units:

1)RCH(OH)CH₃

2)RCOCH₃

*R denotes H, alkyl or aryl group

In positive tests, the reaction will produce a yellow precipitate of CHI₃.

Equations:

*Click on picture for a clearer and bigger view
Any compound which contains the above mentioned structural units will result in a positive iodoform test. Ethanol is the only first degree alcohol to give a positive iodoform test.

Aldehydes and Ketones (carbonyl compounds)

Aldehydes and ketones are carbonyl compounds which undergo Nucleophilic Addition. The oxygen atom bonded to the carbon is electronegative, hence, the C atom becomes slightly positive. The slightly positive C atom is thus the target of nucleophiles. It is easy to remember the reactions that aldehydes and ketones undergo. The most important thing to remember for this chapter is the identification of aldehydes and ketones. This is done so by carrying out a variety of tests which you have learnt in the syllabus. It is also important to be able to draw the compounds that are formed when the respective tests are carried out, especially for Brady's test. I have not included the equations for the Iodoform test clearly in the mindmap below. Do not that the equations of the Iodoform test are just as important to memorise.

Halogenoalkanes (alkyl halides)

Halogenoalkanes, otherwise known as alkyl halides, consists of one or more halide atoms bonded to a carbon. This is represented by R-X in the mindmap, where R is a carbon atom while X is a halide atom. Alkyl halides undergo 2 types of reactions, mainly Elimination and Nucleophilic Substitution. In Nucleophilic substitution( N.S), the halide atom is substituted by a nucleophile. Nucleophiles are molecules/ions with at least a bond pair. Common nucleophiles include cyanide ions, hydroxide ions and ammonia.

Summary of Alkyl Halide reactions:

Hydroxy Compounds

Hydroxy compounds basically have the functional group -OH. The syllabus currently requires us to know the reactions for alcohol and phenol compounds. Alcohol is able to undergo oxidation while phenol is not able to due to strong bonds between -OH and benzene. Do note that the use of a strong or weak oxidising reagent will result in a different compound being formed from alcohol.

Phenol is able to undergo electrophilic substitution (Different from electrophilic addition in alkenes). As the name suggests, electrophilic substitution involves substitution. In phenol's case, a H atom is substituted to give a -OH functional group.

A summary of reactions that an alcohol and phenol undergoes:
As usual, please do not republish the image under your name, give credits.

Alkenes

The first topic i will be doing about is on Chemistry for H2 students. The current subject is being taught at JC levels but it may still help for secondary school students who are studying on Organic Compounds.

Alkenes are unsaturated organic compounds containing at least one carbon-carbon double bond (denoted: C=C). Alkenes have a general formula of CnH2n. Alkenes undergo electrophilic addition as well as oxidation reactions. The term electrophilic addition indicates the addition of an electrophile (a molecule which is attracted to electrons). C=C bonds contain one pi (π) and one sigma (σ) bond. The C in alkenes undergo sp2 hybridisation. Alkenes may also display cis-trans isomerism.

A summary of reactions undergone by alkenes: (click on image to enlarge, you can save it and print for yourself)
Choose the ideal one for viewing. I just happened to find that inverting the colours using paint produces interesting results. =)
P.S: Please do not edit and republish under your name.