Curved Arrows

Overview

Curved arrows represent electron movements.

Chemists utilize curved-arrow (electron-pushing) notation as a bookkeeping tool to keep track of electron movements (bond breaking and formation) within and between compounds. The notation helps in understanding complex reactions, predicting products for an unfamiliar reaction (e.g., on an exam), or even how to exploit a reaction for industrial or medicinal benefits (e.g., selective radiolabeling).

Basic rules of curved-arrow notation

- The arrow starts at an electron source (a bond or a lone pair). In other words, the tail of the curved arrow shows where the electrons originate.
- The arrow points to where the electrons are heading.
- A double-barbed arrow shows the movement of two electrons. A single-barbed arrow shows the movement of one electron. (Below, all examples utilize doubled-barbed arrows.)

Examples

Don't try to memorize these scenarios; just get a solid grasp on reading and interpreting what the curved arrows are trying to convey.

In example 1, A is bonded to B. A curved arrow starts from the bond and points to B. This results in A positively charged and B negatively charged with a lone pair. In example 2, A is negatively charged with a lone pair and B is positively charged. A curved arrow starts from the lone pair of A and points to B. This results in A bonded to B. In example 3, A is negatively charged, has a lone pair and a single bond to B, which is positively charged. A curved arrow starts from the lone pair on A and points to the single bond, resulting in A double bonded to B. Below this, an alternative mechanism for example 3 shows the curved arrow starting from the lone pair on A and pointing to B to form the double bond.

Example 1 starts with A bonded to B reacting with C plus. A curved arrow starts from the single bond and points to C. This results in A bonded to C with a positively charged B ion or B bonded to C with a positively charged A ion. In example 2, A is single bonded to a negatively charged B with a lone pair. A curved arrow starts from the lone pair on B and points to C plus, forming A single bonded to B which is single bonded to C. Example 3 shows another way of making the same product by starting with A single bonded to B plus reacting with C with a lone pair and minus charge. A curved arrow starts on C and points to B plus. Example 4 shows the same reactants as example 3, except there is a second curved arrow starting at the single bond and pointing to A. This results in A anion with a lone pair and B plus single bonded to C. Example 5 shows a positively charged A single bonded to B which is single bonded to a negatively charged C with a lone pair. A curved arrow starts from the lone pair on C and points to A. This results in a 3-membered ring consisting of A, B and C.

Example 1 shows A double bonded to B. A curved arrow starts from the double bond and points to B, resulting in a positively charged A single bonded to a negatively charged B with a lone pair. Example B shows a negatively charged A with a lone pair double bonded to B with a positive charge. The curved arrow starts from the lone pair on A and points to the double bond. This results with A triple bonded to B. Below this, the same example shows alternatively, some books draw the curved arrow starting from the lone pair and pointing to B instead of the bond, giving the same triple bonded product.

Example 1 starts with A double bonded to B. A curved arrow starts from the double bond and points to a positively charged C. This results in two possible products, A single bonded to C and a positively charged B or B single bonded to a positively charged A and single bonded to C. In reaction 2, A double bonded to B reacts with a positively charged C with a lone pair to give a 3-membered ring of A, B and C plus. Two possible mechanisms form this – both start with a curved arrow on the lone pair of C and points to either A or B. The second curved arrow starts from the double bond and points to C. Example 3 shows A double bonded to B with a lone pair. A curved arrow starts from the lone pair and points to C plus, forming A double bonded to B, which has a positive charge and bond to C.

Example 1 shows A double bonded to B, which is single bonded to C plus. A curved arrow starts from the double bond and points to the single bond. This results in a positively charged A single bonded to B, which is double bonded to C. Example 2 shows A double bonded to B, which is single bonded to a negatively charged C with a lone pair. A curved arrow starts from the lone pair and points to the single bond between B and C, forming A double bonded to B double bonded to C, and B has a negative charge. Example 3 shows A double bonded to B, which is single bonded to a negatively charged C with a lone pair. A curved arrow starts from the lone pair and points to the single bond between B and C, and a second curved arrow starts from the double bond and points to A. This form a negatively charged A which has a lone pair and single bond to B, which is double bonded to C.

Page updated

Google Sites

Report abuse