#AdditionReaction

#Chemistry

#OrganicChemistry

Introduction

• Alkenes undergo addition reactions in which atoms of a simple molecule add across the C=C double bond.

• In an addition reaction, only one product is formed.

• But only chemical compounds containing multiple bond character can undergo an addition reaction as a double or triple bond is usually broken to form the required single bonds.

• An addition reaction is essentially a reverse of a decomposition reaction wherein a decomposition reaction is a reaction where one compound decomposes into one or more elements or compounds.

• Looking at an example of an addition reaction, hydrochlorination of propane (an alkene), for which the equation is :

  CH3CH = CH2 + HCl → CH3C+HCH3 + Cl− → CH3CHClCH3

• Addition reactions convert an alkene into an alkane by adding a molecule across the double bond.

  
  

Types of Addition Reaction of Alkenes

• There are four major types of addition reactions that can occur with alkenes, they include: Hydogenation, Halogenation, Hydrohalogenation, and Hydration.

Hydrogenation

• In a Hydrogenation reaction, hydrogen (H2) is added across the double bond, converting an unsaturated molecule into a saturated molecule.

• Note that the word hydrogen is found in this reaction name, making it easier to remember and recognize: Hydrogen-ation. In a hydrogenation reaction, the final product is the saturated alkane.

Halogenation

• In a Halogenation reaction group 7A elements (the halogens) are added across the double bond. The most common halogens that are incorporated include chlorine (Cl2), bromine (Br2), and Iodine (I2).

• Notice that the term halogen is found in this reaction name, making it easier to remember and recognize: Halogen-ation. In halogenation reactions the final product is haloalkane.

Hydrohalogenation

• In Hydrohalogenation, alkenes react with molecules that contain one hydrogen and one halogen. Hence the name Hydro–Halogen-ation.

• HCl and HBr are common hydrohalogens seen in this reaction type. In hydrohalogenation, the hydrohalogen is a polar molecule, unlike the nonpolar molecules observed in the halogenation and hydrogenation reactions.

• In the case of the hydrohalogen, the end of the molecule containing hydrogen is partially positive, while the end of the molecule containing the halogen is partially negative.

• Thus, when the negatively charged electron from the alkene double bond attacks the hydrohalogen, it will preferentially attack the hydrogen side of the molecule, since the electron will be attracted to the partial positive charge.

• The halogen will then form the negatively charged anion observed in the intermediate structure and attach second during the addition reaction. The final product is a haloalkane.

Hydration

• Just like when your are feeling thirsty, the terms hydration and dehydration refer to water.

• Hydration means the addition of water to a molecule, just like when you feel fully hydrated or full of water, while dehydration means the removal or elimination of water, just as when you are feeling dehydrated and need some water to drink. Similar to the hydrohalogenation reaction above, water is also a polar molecule.

• In this case, the water is split into two groups to be added across the double bond of the alkene.

• It is split into the H- and the -OH components. Similar to the hydrohalogenation reaction, the hydrogen adds first, as it carries the partial positive charge.

• The OH group forms the negative anion intermediate and is then added to the carbocation to form the final product, which is an alcohol.

Types of Addition Reaction

• An addition reaction is a reaction in which an atom or molecule is added to an unsaturated molecule, making a single product. An addition reaction can be thought of as adding a molecule across the double bond or triple bond of an alkene or alkyne.

• Alkenes primarily undergo electrophilic addition reactions, where the double bond reacts with electrophiles, such as hydrogen halides and water.

• For polar addition reactions there are two classifications, namely: Electrophilic Addition reactions and Nucleophilic Addition reactions.

• For non-polar addition reactions, we have two classifications, namely: Free radical addition reactions and Cycloadditions reactions.

• An electrophilic addition reaction can be described as an addition reaction in which a reactant with multiple bonds as in a double or triple bond undergoes its π bond broken and two new σ bonds are formed.

• In chemistry, pi bonds (π bonds) are covalent chemical bonds, in each of which two lobes of an orbital on one atom overlap with two lobes of an orbital on another atom, and in which this overlap occurs laterally.

• Sigma bonds form by the direct head to head overlap of atomic orbitals and can occur between s-s, p-p and s-p orbitals. Pi bonds form by the side to side overlap of p orbitals. (Will Make another Post).

• A nucleophilic addition reaction is an addition reaction where a chemical compound with an electron-deficient or electrophilic double or triple bond, a π bond, reacts with a nucleophile which is an electron-rich reactant with the disappearance of the double bond and creation of two new single, or σ, bonds.

• Alkanes cannot undergo nucleophilic addition reactions because they are already structurally stable and cannot become stronger or more stable. Alkanes only have single σ-bonds, so they can only switch things around in substitution reactions.