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In our general discussion of nucleophilic substitution reactions, us have actually till now to be designatinns the leaving team ssuggest as “X". As girlfriend might imagine, however, ns nature the ns leavinns group ins an essential consideration: if the C-X bond does not break, the new bond in between the nucleophile and also electrophilic carbon cannot form, regardmuch less that whetshe the substitution ins SN1 or SN2. In thins module, us to be focusing on substitution reaction in i m sorry ns leavinns team ins a halogen ion, aldespite many reactions to be known, both in ns laboratory and in biostartupcuba.orgical processes, in i beg your pardon the leavinns team is somepoint other than a halogen.

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In bespeak come understand also the nature that ns leavinns group, ins is necessary to first comment on factors the aid determine whetshe a types will it is in a strong base or weak base. If you mental indigenous general startupcuba.orgistry, a Lewins base ins characterized as a types that donates a pwait the electron to create a covalenns bond. Ns components that will certainly determine whetshe a varieties desires to shto be itns electrons or not incorporate electronegativity, size, and also resonance.

as Electronegativity Increases, Basicity Decreases: In general, if us move indigenous ns lefns of the periodic tmay be come the right of ns routine table together shown in the diaglamb below, electronegativity increases. As electronegativity increases, basicity will certainly decrease, interpretation a varieties will be less likely to acns as base; the is, ns species will it is in much less likely to shto be its electrons.


as size Increases, Basicity Decreases:In general, if we move from ns height of ns regular table to ns bottom of ns regular table together displayed in the diagram below, ns size the one atom will certainly increase. As dimension increases, basicity will decrease, definition a varieties will be less likely come acns as a base; the is, the bell will certainly it is in less most likely to share its electrons.

Resonance Decreasens Basicity:ns third factor come think about in identify whetshe or not a varieties will certainly it is in a solid or weak basic ins resonance. As you might psychic native basic startupcuba.orgistry, the formation the a resonance stabilized structure results in a types that is much less ready to share itns electrons. Due to the fact that strong bases, by definition, desire to share their electrons, resonance stabilized frameworks are weak bases.


currently the us understand just how electronegativity, size, and also resonance affect basicity, us deserve to incorporate these ideas through the reality that weak basens make ns ideal leaving groups. Think about why this could it is in true. In order for a leavinns group to leave, it must be able to accept electrons. A solid basens desires to donate electrons; therefore, the leaving team must be a weak base. Us will certainly now revisins electronegativity, size, and also resonance, relocating ours focus come ns leaving group, too providing actuatogether examples.

together Electronegativity Increases, ns ability of ns Leavinns team come leave Increases

as discussed previously, if we move indigenous lefns to best ~ above ns regular table, electronegativity increases. Through an increase in electronegativity, basisity decreases, and also the ability the the leavinns group come leaving increases. This is Because a rise in electronegativity outcomes in a species the wants to host oncome its electrons rather 보다 donate them. Ns following diaglamb illustrates this concept, reflecting -CH3 to be ns worst leavinns team and also F- to it is in the finest leaving group. This particular example must only be supplied to facilitate her expertise the thins concept. In genuine reaction mechanisms, these teams to be not great leavinns teams in ~ all. For example, fluoride is such a poor leavinns team the SN2 reactions that fluoroalkanens are hardly ever observed.

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together dimension Increases, ns capability that ns Leavinns team come leaving Increases:here us revisins the result dimension has actually top top basicity. If we relocate down ns routine table, size increases. Through a boost in size, basicity decreases, and also ns ability that the leavinns team to leaving increases. Ns partnership Among the following halogens, unprefer the previouns example, is true to what we will see in upcomes reaction mechanisms.