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Minggu, 30 September 2012

Reactions related to the Hydrocarbon, Alkanes, Alkenes, and Alkynes

Reactions of Hydrocarbon

In the compounds of hydrocarbons (alkanes, alkenes, alkynes) reactions can occur, such as oxidation reactions, addition reactions, substitution reactions and elimination reactions. In this section, you will study these reactions.
1. Oxidation Reactions in Hydrocarbon Compounds
An alkane compounds that react with oxygen to produce carbon dioxide and water is called a combustion reaction. Consider the equation following oxidation of hydrocarbons.

Combustion reaction, basically an oxidation reaction. In the compound methane (CH4) and carbon dioxide (CO2) contains one carbon atom. Both of these compounds should have the zero oxidation oxidation 

number of carbon atoms in the compound methane is -4, while the oxidation number of carbon atoms in the compound carbon dioxide is +4.
Oxidation number of C atoms in the compound increased carbon dioxide (oxidation), whereas oxidation of methane C atoms in the compound decreases.

2. Substitution Reactions in Hydrocarbon Compounds
Substitution reaction is a substitution reaction of functional groups (atoms or molecules) attached to the C atom of a hydrocarbon compound. In reaction to halogenated alkanes, the hydrogen atoms attached to the C atom substituted alkane compounds with halogen atoms. When a mixture of methane and chlorine heated to 100 ° C or by UV radiation are produced klorometana compounds, such as the following reaction.

If chlorine gas is still available in the mixture, the reaction will continue as follows.

Substitution reactions are used in the manufacture of compound dichloromethane. If the reaction is carried out on the compound ethane, the reaction will produce dikloroetana. Dichloromethane used for flaking paint, while triklorometana used to dry-clean.

3. Addition Reactions in Hydrocarbon Compounds
If a compound has carbon double bonds (alkenes) or triple (alkyne) and the carbon atoms are reduced double bond, then replaced with functional groups (atoms or molecules). The reaction is called an addition reaction. Consider the reaction between 1-propene with hydrogen bromide produced 2-bromopropane as follows.

Hydrocarbons with double bonds or triple an unsaturated compound. In unsaturated compounds allows for the addition of a hydrogen atom. When an unsaturated compound is reacted with hydrogen halides will produce a single product.
Markovnikov rule: the addition of acid to an alkene symmetry, the H atom will bind more H atoms.

4. Elimination Reactions in Hydrocarbon Compounds
Elimination reaction is the reverse reaction of the addition reaction. Elimination reaction involving the release of atom or group of atoms of a molecule to form a new molecule. An example is the elimination reaction producing ethene elimination of ethyl chloride and hydrochloric acid.

Elimination reaction occurs in saturated compounds (no double bonds) and produces unsaturated compounds (having a double bond)

 Reaction of Alkanes

Why are alkanes unreactive ?

Alkane = paraffin (parum affinis = little affinity)

• Electrons between C and H distributed evenly.
• None of the atoms of a significant charge.
• Alkanes have no electrophile or nucleophile.
• In normal conditions can not react with a strong acid (H2SO4), oxidizing (Br2, O2, KMnO4)


•    At high temperatures all saturated hydrocarbons (alkanes) can react with oxygen (combustion reaction).
•    If the amount of oxygen is quite excessive, then the combustion reaction can take place perfectly with the burning form of H2O and CO2.


    Cracking is the reaction of the alkane compound solution to generated smaller compounds.

  • Cracking process is usually carried out under conditions that allow gasoline produced as much as possible.
  • In the hydrocracking process, hydrogen is added to produce saturated hydrocarbons
  • Cracking without the hydrogen will produce a mixture of alkanes and alkenes.

Alkanes can react with halogens (F2, Cl2, Br2, I2) produces alkilhalida.

• The order of reactivity: F2> Cl2> Br2> I2
• reaction with I2 is very slow.
• The reaction of alkanes with Cl2 and Br2 lasted not so easy to control how fast
• The reaction of alkanes with F2 so rapid that it is difficult to control.

Solving homolisis the chlorine molecule into two chlorine atoms, which is caused by blue
radiation (given the symbol hv).

Chlorine atom has an unpaired electron, and acts as a free radical.

    Why blue ray can break Cl2?
  • To break down the molecular chlorine (Cl 2) to 2 atoms of chlorine (Cl) energy required for 242 kJ / mol.
  • The energy of a photon beam can be calculated using the equation: E = hv = 250 kJ per mole of photons
          With      h  =  Planck' constant
                         v =  frequency


When chlorine radicals collide with the molecules of methane, one hydrogen atom from methane drawn, so the resulting HCl and methyl radicals.

Metal radicals react with molecular chlorine to form chlorine radicals klorometana and new. This chlorine radicals will return moleul collide and react with methane, forming HCl and methyl radicals. And so will be a chain reaction.


      Recombination of two radicals:

Overall reaction:

 Reactions of Alkenes

Why is alkene reactive ?

The electrons in the pi bond arespread farther from the carbon nuclei and are more loosely held than the sigma electrons.

Why the double bond can react? Basically, organic chemistry discusses the interaction between the atoms of the excess electron with the atom-atom electron deficient. This interaction is in the form of gravity that allows the chemical reactions. There needs to be rules to determine the reactivity of organic compounds: atom or molecule will be attracted to the excess electron atoms or molecules that lack an electron. To understand how a functional group can react, we must recognize that excess atoms or molecules or atoms electrons and electron-deficient molecules.

Alkene compounds reactivity caused by a double bond (π bond).   bond is a location with a high electron density, so it is a nucleophile. Since  bond is more stable than the π bond, the most common reaction is to change the alkene compounds π bond into  bond. Reaction that happens is ELECTROPHILIC ADDITION reactions.


1.    Electrophilic Addition of Hydrogen Halide

1. Electrons in the alkene π bond attracted by H+ from HBr, thus breaking the bond HBr Br carry bonding electrons.
2. Electrons are just "ripped" from a carbon atom, and is still attached to the other carbon atom.
3. Two electrons that had been forming π bond, now form a new σ bond between carbon and hydrogen from HBr.
4. The result of this step is a carbocation, because C does not form a new bond with H no longer share electrons, making it positively charged.

5. Lone pair on Br-ion to form a bond with the positively charged carbon of the carbocation, forming the compound 2-bromobutana.

1.Make certain that the arrows are drawn in the direction of the electron flow and never against the flow. This means that an arrow will always be drawn away from a negative charge and/or toward a positive charge.

2.Curved arrows are drawn to indicate the movement of electrons. Never use a curved arrow to indicate the movement of an atom. For example, you can’t use an arrow as a lasso to remove the proton, as shown here:

3.The arrow starts at the electron source. It does not start at an atom. In the following example, the arrow starts at the electrons of the bond, not at a carbon atom:

Examples of electrophilic addition reactions:
CH2 = CH2+ HCl   ->  CH3CH2Cl
    etena                           etil klorida

2.    Free-radical Addition of Hydrogen Bromide
In 1933, M. S. Kharasch and F. W. Mayo studied addition reactions of HBr to alkenes in the presence of peroxide compounds (ANTI Markovnikov). Peroxide will generate free radicals that acts as a catalyst to accelerate the addition reaction, causing the reaction proceeds through a different mechanism. The oxygens bond in the peroxide weaker so it can be split into two radicals.


3.    Water Addition (HYDRATION)

If water is added to the alkene compounds, there would be no reaction because there is no reaction with electrophiles who started alkena.Ikatan O  H in water is too strong to release water  H as electrophiles acid is very weak.

•    Apabila kedalam campuran ditambahkan asam (misal HCl), maka akan terjadi reaksi karena asam dapat menyediakan elektrofil.

•    H2SO4 (pKa = -5) and HCl (pK a = -7) is a strong acid, so it can be dissociated perfect.
Acid participating in the reaction are a hydronium ion (H3O +)

REACTION MECHANISM (follow the Markovnokov rules)

As with water, alcohol can react with alkenes in the presence of acid, resulting in ether.

    Some electrophilic addition reaction products which are not the result of the addition of electrophiles to the carbon sp2 which has particularly more hydrogen amount and nucleophile addition to other sp2 carbon atom. For example HBr addition to 3-methyl-1-butene will pay 2-bromo-3metilbutana (slightly) and 2-bromo-2-methylbutane (primary outcome).

Another example is the addition of HCl pada3,3-dimethyl-1-butene would produce 3-chloro-2,2-dimetilbutana (expected product) and 2-chloro-2 ,3-dimetilbutana (unexpected product). Again, unexpected product became the main product.

       F. C. Whitmore is the man who first explained why the product is not expected to dominate it. This was due to the rearrangement of the carbocation intermediate. Carbocation rearrangements undergo rearrangements only when it becomes more stable.
    For example if an electrophile mengadisi on 3-methyl-1-butene, it will initially formed secondary carbocation. Secondary carbocation has hydrogen can shift with a pair of electrons to the carbon next to a positively charged, so it will be more stable tertiary carbocation.

The results of the two carbocation rearrangment alkilhalida: one is the result of the addition of nucleophiles in unrearranged carbocation and the addition of the rearranged carbocation.

The main products are derived from the rearranged carbocation.

Because hydrogen shift rearrangement was involved with a pair of electrons, so the shift is called a hydride shift (H : -is a hydride ion).

In particular, the shift is called 1,2-hydride shift, because the hydride ion moves from one carbon atom to another carbon atom next to them. (note that this does not mean the hydride moves from C-1 to C-2.)

Question :

  1. reaction to anything related to Hydrocarbon, alkanes, alkenes, and alkynes are present in our daily lives and say what are the functions of these reactions

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