If you don't, you risk losing marks in an exam.

However, it is essential that you show the lone pair you are interested in as a pair of dots.

These aren't normally shown because they don't actually do anything new and interesting! Note: There are another three lone pairs around the outside of the bromide ion - making four in all. You can think of the electrons shown on the bromide ion as being the ones which originally made up the hydrogen-bromine bond. The first stage leaves you with a positive charge on the right hand carbon atom and a negative bromide ion. The second stage of this reaction nicely illustrates how you use a curly arrow if a lone pair of electrons is involved. You have to show electron pair movements as curly arrows - not as straight ones. Notice also that the electron movement between the H and Br is shown as a curly arrow even though the electron pair moves straight down. Notice that the arrow head points between the C and H because that's where the electron pair ends up. Drawing the bond as a line is enough, but you could put two dots in as well if you wanted to. There's no need to draw the pairs of electrons in the bonds as two dots. At the same time the pair of electrons in the hydrogen-bromine bond moves down on to the bromine atom. Those electrons move to form a new bond with the hydrogen from the HBr. The arrow head is where you want the electron pair to end up.įor example, in the reaction between ethene and hydrogen bromide, one of the two bonds between the two carbon atoms breaks. Remember that a lone pair is a pair of electrons at the bonding level which isn't currently being used to join on to anything else. That's always fairly obvious, but you must show the electron pair either as a bond or, if it is a lone pair, as a pair of dots. The arrow tail is where the electron pair starts from. You mustn't use them for any other purpose. Using curly arrows to show the movement of electron pairsĬurly arrows (and that's exactly what they are called!) are used in mechanisms to show the various electron pairs moving around. You can jump straight to the movement of single electrons further down this page if that is all you are interested in for the moment (for example, if you are currently working on free radical reactions). This page explains the use of curly arrows to show the movement both of electron pairs and of single electrons during organic reaction mechanisms.

USING CURLY ARROWS IN REACTION MECHANISMS The use of curly arrows to show electron movements in reaction mechanisms

Bromination mechanism chem draw