This article is written to address the topic that starts causing lots of confusion when the explanation deviates from the “in S_N2, a wedge becomes a dash, and a dash becomes a wedge” statement. So, let’s start with this statement then. What does it mean when they say in S_N2, a wedge becomes a dash, and a dash becomes a wedge?

A secondary alkyl halide is chiral at the carbon bearing the halogen; this halogen must be shown by a wedge or a dashed line. For example, in (S)-2-bromobutane, the Br must be shown with a wedge line when drawn in this orientation (remember wedge and dash is relative and it depends on the direction we are looking at):

 

 

Now, when this molecule is reacted with a good nucleophile such as cyanide ion, the CN replaces the Br, but it is now shown with a dashed line:

 

 

This is because of the mechanism of the S_N2 reaction: The nucleophile attacks from the opposite side of the leaving group (backside attack).

So, when the leaving group is wedge pointing towards us, the nucleophile attacks from behind the plane, thus appearing in the product as a dash. If the leaving group is dash, then the nucleophile attacks from our side and appears as a wedge in the product:

 

 

This pattern indicates that the S_N2 reaction is stereospecific – the stereochemistry of the product is dependent on the stereochemistry of the starting material.

This feature of the S_N2 mechanism is explained by the molecular orbitals (MO theory) of the nucleophile and the electrophile.

According to this theory, the lone pair(s) of the nucleophile is located in the highest occupied molecular orbital (HOMO) and they flow to the lowest unoccupied molecular orbital (LUMO) of the electrophile. The LUMO is the anti-bonding orbital of the C-Halogen σ bond (σ*).

It is the lowest in energy among all the empty orbitals, and therefore, the lone pair of the nucleophile is directed there:

 

 

The geometry of the orbitals is such that the antibonding orbital of the C-Br bond is right across the bonding orbital so the only way of occupying this orbital by the lone pairs of the nucleophile is when it attacks at 180^o to the leaving group (back-side attack):

 

 

More on Wedge and Dash in S_N2 Reactions 

The wedge and dash notation is handy for a quick guide; however, you need to keep in mind that it is not an ultimate indicator of the reaction mechanism or the configuration of an atom. Wedge and dash simply depend on the direction we are looking from:

 

 

Keep an eye out for tricky questions using the wedge and dash notation.

For example, does the following reaction go by an S_N2 or S_N1 mechanism?

 

 

The thing here is that you may look at it and think that since the wedge leaving group is changed into a dash nucleophile, then it must be S_N2!?

You should notice, however, that the carbon connected to the leaving group is not chiral since the molecule has a plane of symmetry and the dash line becomes wedge by simply flipping the molecule 180^o:

 

 

Therefore, sometimes we cannot determine if the reaction is S_N2 simply by looking at the wedge and dash lines. In fact, this is a reaction of a secondary alkyl halide with a weak nucleophile and is likely to go by an S_N1 mechanism.

Since there is no chirality center, the wedge and dash lines should be normally replaced with plain lines. 

 

 

The take-home from this example is not to decide on the mechanism by only comparing the wedge and dash notation of the leaving group and the nucleophile. 

 

S_N2 Reactions of Achiral Substrates

Let’s also consider what happens if, instead of the alcohol, its conjugate base alkoxide ion is reacted with chlorocyclihexane. Remember that alkoxide ions are good nucleophiles and strong bases; therefore, we are going to decide between S_N2 and E2 mechanisms. In most cases, the predominant path of reacting secondary alkyl halides with basic good nucleophiles such as alkoxides is the E2 mechanism.

 

 

However, the S_N2 product is also often observed, and for the purpose of today’s discussion, we will focus on that. You can read more on deciding between S_N1, S_N2, E1, and E2 mechanisms here.

The product is the same ether as we saw in the S_N1 reaction with the alcohol. However, it follows an S_N2 mechanism; however, the wedge and dash notations are still sort of irrelevant because the substrate is achiral. Now, why do we say “sort of” irrelevant?

It is important to mention that the mechanism is S_N2, which means the nucleophile attacks from the opposite side of the leaving, and thus their “physical positions” in the transition state are different. However, it has no significance in terms of the wedge and dash and even R and S notation because both the substrate and the product are achiral (not chiral).

We can perhaps see this better by drawing the cyclohexane in a chair conformation:

 

 

Let’s also consider a similar reaction where the substrate and the product are achiral, but the wedge and dash notation does matter:

 

 

The carbon connected to the leaving group is, again, not chiral, and the molecule has a plane of symmetry. However, in this case, we do need to show the wedge and dash lines, and it is not to make it tricky.

The difference with the previous reaction is that there is a methyl group here pointing towards us, and depending on whether the nucleophile appears as a wedge or dash, we get two different products:

 

 

These are cis and trans isomers – a pair of diastereomers and, of course, you can’t do the 180^o flip again.

Now, if the reaction goes by an S_N2 mechanism, then the nucleophile should appear as a dash – anti to the methyl group since it attacks from the opposite side of the leaving group:

 

 

The S_N1 reaction would have resulted in a pair of diastereomers since it forms a carbocation intermediate before the nucleophilic attack:

 

 

Notice that in any case, the configuration of the methyl group does not change as it is not part of the reaction.

 

S_N2 With Retention of Configuration

As we discussed above, the only sure way of determining whether there was an inversion of configuration or not is by comparing the absolute configuration of the starting material and the product (R and S).

But what if you see a reaction when the absolute configuration of the ɑ-carbon stays the same?

 

 

How do you know if this is S_N2 or S_N1, since we know that the configuration changes from R to S or S to R if it is S_N2, and it racemizes in S_N1 reactions (check the stereochemistry of S_N1 reactions here)?

Well, the S_N1 is out since the product is shown as a single enantiomer, meaning that the other enantiomer is not formed.

And the question is, how do you perform an S_N2 and keep the configuration the same?

The starting material here is S, and an S_N2 reaction would change it to R. However, if we do two S_N2 reactions it will go S-R-S:

 

 

One approach for achieving this would be converting the OH into a halide, followed by a reaction with sodium sulfide. There are different ways of converting an alcohol into a good leaving group, and you may not be familiar with some of them now:

 

 

The method shown above uses the mesylation of alcohols to convert them into a good leaving group, which can then be expelled by the Br^– and undergoes another S_N2 reaction with the sulfide ion.

This was a three-step synthesis applying double S_N2 to retain the absolute configuration, but:

 

Can an S_N2 reaction process occur with retention of configuration?

This is rather an exception that you will likely not need to deal with, but yes, you can have an S_N2 reaction where the absolute configuration is not changing.

For example, in the following reaction, the absolute configuration does not change even though it goes through an S_N2 mechanism:

 

 

The reason for this is that, unlike the leaving group, the nucleophile is not the highest priority on the chiral carbon:

 

 

Primary and secondary alcohols can also serve as the electrophile in an S_N2 reaction, by converting the OH into a good leaving group such as a mesylate, tosylate, or triflate, or by using the Mitsunobu reaction, which allows this transformation in one-pot synthesis:

 

 

There are other tricky situations that you will encounter in S_N2 and S_N1 reactions, and we have a separate post discussing them, so feel free to check that out as well.

 

Check Also

• Introduction to Alkyl Halides
• Nomenclature of Alkyl Halides
• Substitution and Elimination Reactions
• Nucleophilic Substitution Reactions – An Introduction
• All You Need to Know About the S_N2 Reaction Mechanism
  The S_N2 Mechanism: Kinetics, Thermodynamics, Curved Arrows, and Stereochemistry with Practice Problems
• Stability of Carbocations
• The S_N1 Nucleophilic Substitution Reaction
• Reactions of Alkyl Halides with Water
• The Stereochemistry of the S_N1 Reaction Mechanism
• The S_N1 Mechanism: Kinetics, Thermodynamics, Curved Arrows, and Stereochemistry with Practice Problems
• The Substrate and Nucleophile in S_N2 and S_N1 Reactions
• Carbocation Rearrangements in S_N1 Reactions with Practice Problems
• Ring Expansion Rearrangements
• Ring Contraction Rearrangements
• When Is the Mechanism S_N1 or S_N2?
• Reactions of Alcohols with HCl, HBr, and HI Acids
• SOCl₂ and PBr₃ for Conversion of Alcohols to Alkyl Halides
• Alcohols in S_N1 and S_N2 Reactions 
• How to Choose Molecules for Doing S_N2 and S_N1 Synthesis-Practice Problems
• Exceptions in S_N2 and S_N1 Reactions
• Nucleophilic Substitution and Elimination Practice Quiz
• Reactions Map of Alkyl Halides