Bromine and ethene, step by step

Section 4 of 4

Bromine adds across ethene's C=C double bond and makes 1,2-⁠dibromoethane, with one bromine on each carbon. Curly arrows show how, in two steps.

The same first step is behind the bromine water test in lesson 8, where orange bromine water turns colourless. Ethene's double bond is electron-rich. A bromine molecule, Br–⁠Br, shares its pair evenly. As it comes close to the C=C, the crowd of electrons in the double bond pushes that pair away, toward the far end. The nearer bromine becomes δ+ and the far one δ−.

Step 1

Arrow 1 starts on the C=C double bond and points to the δ+ bromine: the second pair of the double bond becomes a new C–⁠Br bond. A bromine atom's outer shell is full with 8 electrons, so as the new bond forms, the Br–⁠Br bond breaks. Arrow 2 carries its pair to the far bromine, which leaves as a bromide ion, Br⁻.

Step 1: the C=C double bond reaches out to a bromine molecule Top: ethene, two carbon atoms joined by a double bond with two hydrogen atoms on each, and a bromine molecule, Br-Br, standing upright above the double bond. The lower bromine is marked delta plus and the upper one delta minus. Curly arrow 1 starts on one line of the double bond, which is amber, and points up to the lower bromine. Curly arrow 2 starts on the amber Br-Br bond and points to the upper bromine. A straight arrow points down to the products: a bromide ion, Br minus, holding an amber lone pair, and a carbocation, a CH2 group with a plus charge joined to a CH2 group that carries the first bromine atom on a new amber bond. CCHHHHBrBrCCBrHHHHBr δ+ δ− ethene bromine 1 2 − + bromide ion carbocation
Step 1. Arrow 1 takes a pair from the C=C double bond (amber) to the nearer bromine and makes a new C–⁠Br bond (amber). Arrow 2 takes the Br–⁠Br pair (amber) to the far bromine, which leaves as a bromide ion.

The pair that went to bromine was shared by both carbons. The left carbon has lost its share, so it has only three bonds and a + charge. A carbon with a + charge like this is a carbocation, and it is very electron-poor.

Step 2

The bromide ion is a nucleophile, with lone pairs and a − charge, and the carbocation is an electrophile. One arrow carries a lone pair from the bromide ion to the + carbon and makes the second C–⁠Br bond. The + and − charges cancel, and the product is 1,2-⁠dibromoethane.

Step 2: the bromide ion joins the carbocation Top: a bromide ion, Br minus, with an amber lone pair facing the plus carbon of the carbocation, a CH2 group joined to a CH2 group that carries a bromine atom. A curly arrow starts on the lone pair and points to the plus carbon. A straight arrow points down to 1,2-dibromoethane, Br-CH2-CH2-Br, with the new C-Br bond, made from the lone pair, in amber. CCBrHHHHBrCCBrHHHHBr − + bromide ion carbocation new bond 1,2-⁠dibromoethane
Step 2. The bromide ion's lone pair joins the + carbon, and the new C–⁠Br bond (amber) completes 1,2-⁠dibromoethane, BrCH₂CH₂Br.
1,2-Dibromoethane, C₂H₄Br₂Two carbon atoms joined by a single bond, each also bonded to one bromine atom and two hydrogen atoms.BrBrCCHHHH
1,2-⁠Dibromoethane, C₂H₄Br₂, the product: one bromine atom on each carbon, joined by a single bond.

Quick check

In step 1, which electrons does arrow 1 move?

That's the end of lesson 21

In this lesson you found out:

  • What a curly arrow shows, from its tail to its head
  • Why electrons flow from electron-rich places to electron-poor ones
  • How a base takes H⁺ from an acid, in two arrows
  • How bromine adds to ethene, step by step
Start Lesson 22: Separating mixtures All lessons