Understanding The Alkene Hydrohalogenation Mechanism Taking Place In Organic Chemistry Reactions

Alkenes hydrohalogenation involves forming a carbon double bond by breaking a single bond, which is again followed by halogen having hydrogen added through an electrophilic addition. Then the substituted carbon will have a halide added with the help of Markovnikov’s rule. The result obtained is a haloalkane product known as alkyl halide.

Alkene Hydrohalogenation Mechanism Taking Place

Brief summary of Hydrohalogenation Mechanism

Nucleophilic pi bond targets for the electrophilic H bond in the H-X reaction, which results in breaking of the pi bond. Through Markovnikov’s rule, H adds up to the less substituted atom of carbon. This causes a deficit of +1 charge in the more substituted atom of carbon. This provides room for negative halide to form a bond by attacking carbcocation. Finally, what is left is a haloalkane called alkyl halide. Keep in mind that an inert solvent is a must to carry out the entire reaction.

Electrophilic Addition

Detailed explanation of Hydrohalogenation Mechanism


MECHANISM FOR REACTION OF ALKENES WITH HBrThe H-X molecules observed around such as H-Cl, H-Br, and H-I are absolutely high polar. This means that you can expect the electrons between hydrogen and halogen getting hogged because of the tendency of halogen that is highly electronegative. Hence, halogen becomes partially negative, making hydrogen partially positive. Because the pi bond is present at extreme ends of carbon skeleton, they are conveniently distracted by other neighboring molecules. And since, electrons get attracted towards the positive charge, they will possibly look out and reach the electrophile passing by that is positive or even partially positive.

Markovnikov's rule

As soon as an H-X bond passes by alkene, pi bonds get attracted towards hydrogen atom that is partially positive. To trade for the hydrogen atom, out of two any pi electron has to let go off the carbon atom already present in the bond. Now by applying Markovnikov’s rule, carbocation takes place with the more substituted carbon atom. Now since the less substituted carbon atoms haven’t released the electrons, they are singly bound to hydrogen. However, hydrogen can form only a single carbon bond. Hence, halogen acquires the electrons from less substituted carbon atom, and gets floated away in a solution full of negative charge as well as complete octet.


Remember, negative halogens are known as nucleophiles and they get attracted towards electrophiles that are positive. Hence the alone halogen loses electron pairs to attract carbocation forming a sigma bond. Because carbocation took place with more substituted carbon, halogen winds itself with that carbon atom.

alkyl bromide

Solvents play a big role in dissolving reagents as well as reactants without actually hindering the actual process. For this particular reaction we use non polar protic inert solvents such as CCl4, CHCl2, and more. When halide breaks away from hydrogen, the role of halide comes into play. A cage like thing is formed all over negative halogen when the solvent makes use of its partially positively charged hydrogen atoms. Hence, carbocation is prevented by letting off halogen. In fact, what happens is a one of the solvent molecules uses electrons in order to confront carbocation.

Reaction of Alkenes with HBr (radical)


Still have a lot of doubts on how the whole process is taking place? Well, you can surely consult your online chemistry tutoring firm, or an independent online chemistry tutor to help you out in clearing all doubts related to chemistry homework help.