Ionic Substitution SN1 Note all information example problems and graphics taken from lecture Chemistry 14D Thinkbook for Winter 2005 and Organic Chemistry by Paula Yurkanis Bruice 4th ed What does SN1 stand for S for a substitution reaction N for nucleophilic and the 1 for unimolecular meaning that the rate is dependent upon one factor SN1 is a mechanism pathway very similar to SN2 First let s briefly review SN2 SN2 is substitution nucleophilic bimolecular In SN2 the general mechanism is The nucleophile attacks the electrophile from the backside expelling a leaving group Recall that SN2 is a concerted reaction which means all the bond change events takes place at the same time Now let s try making the reaction occur in two steps What if nucleophilic attack occurred in the first step creating a CR3 nucleophile leaving group intermediate and then the leaving group was expelled in the second step It would look like this nuc stands for nucleophile This would absolutely not occur It would create a pentavalent carbon So let s try another approach What if the leaving group left in the first step and in the second step the nucleophile attacked It would look like this Here the leaving group leaves creating a positive charge on the carbon The nucleophile attacks at this site of positive charge This mechanism seems plausible no rules are violated It turns out that this intermediate structure with a positive charge on carbon is crucial to the SN1 mechanism This is called a carbocation because it is a combination of a carbon and a cation Carbocations have incomplete octets so there is a formal positive charge on the carbon This positive charge makes carbocations unstable so carbocations are very reactive Thus carbocations react in a way that will complete the open octet Let s address the issue of carbocations before proceeding further with the SN1 mechanism Carbocations General Structure Carbocations are sp2 hybridized which means that they are in a planar arrangement The pz orbital is empty so you can think of the positive charge as located here Stability The stability of a carbocation depends on two factors number of substituents resonance First we ll look at the number of substituents Remember the positive charge and open octet on the carbon make carbocations unstable Let s look at an example Methyl carbocation primary secondary tertiary Alkyl groups are weakly electron donating So in the above example the more CH3 groups there are the more electron density can be donated to carbon This added electron density somewhat reduces the electron deficiency on the carbon reducing the magnitude of the positive charge and making the carbocation more stable Following the above reasoning methyl carbocations are so unstable that we usually will not encounter them Generalizing the above analysis we can rank from least stable to most stable carbocation Methyl Now let s consider resonance primary secondary tertiary In this example the lone pairs of the pi bond can re form the pi bond between the C and adjacent C This creates a resonance contributor The two resonance contributors together delocalize the positive charge by spreading it between the two carbons This stabilizes the carbocation Allyl carbocation benzyl carbocation allyl and benzyl carbocations have a lot of stability The allyl carbocation has resonance structures because of the pi bond and the benzyl carbocation is even more stable because of resonance involving the attached benzene group Resonance can also occur if an atom attached to the carbocation has lone pairs The lone pairs move from the attached group X to the single bond between X and C creating a double bond This creates an electron deficiency on X moving the positive charge to X Again this spreads out the positive charge over two molecules increasing the stability of the carbocation For example The most electronegative atom oxygen has a positive charge in the second resonance contributor This would normally destabilize the molecule the positive charge should be on the least electronegative atom However all the atoms have full octets so resonance does stabilize the molecule Between degree of substitution and resonance resonance is usually more important For example a primary carbocation with resonance is more stable than a secondary carbocation without resonance primary with resonance secondary without resonance Let s try an example Choose the most stable carbocation primary tertiary w res tertiary secondary Tertiary the most stable arrangement of substituents with resonance is the most stable The Three Fates of a Carbocation Again remember that the driving force for carbocations to react arises from their open octet and positive charge so they will react in a way that allows them to gain electrons Generally carbocations can react in three different ways Let s look at an example to illustrate them 1 capture a nucleophile Here the nucleophile HOCH3 attaches to the carbon bearing the positive charge This fills the open octet on carbon However now oxygen has three bonds so it carries the positive charge Again the positive charge is now on the more electronegative atom but all atoms have a full octet so this is a preferable arrangement 2 lose a proton to form a pi bond Here the electrons on the O of HOCH3 form a bond with a proton on the carbocation pulling it away The electrons of the former proton carbon bond now go into making a pi bond next to the positively charged carbon Now that carbon has four bonds it has a complete octet and the carbocation is stabilized The positive charge has been transferred to CH3OH2 Note that carbocations are so reactive they are desperate that is they can be deprotonated by many molecules that are not particularly good nucleophiles such as water 3 rearrangement or Whenever you see a carbocation always check to see if rearrangement is possible Carbocations will rearrange to improve stability even though the positive charge and open octet will be retained Any C C or C H bond next to the positively charged carbon can be moved Here the first carbocation goes from secondary to tertiary and the second goes from secondary to tertiary with resonance both increase in stability Carbocations are very unselective They often can undergo more than one of these fates and if you see carbocations involved in a reaction you may get different products Now we are ready to present the general mechanism for SN1 Mechanism for SN1 This is a multi step reaction first the leaving