IBDP Chemistry: Nucleophilic Substitution

Let's revisit the definition of basic terms in IBDP organic chemistry!

• Nucleophile: electron-rich chemical species that forms bonds by donating an electron pair
• Electrophile: electron-deficient chemical species that forms bonds by accepting an electron pair
• Nucleophilic substitution: chemical reactions in which an electron-rich chemical species (nucleophile) replaces a functional group within another electron-deficient molecule (electrophile)

Nuc: + R-LG → R-Nuc +LG:

• The electron pair (:) from the nucleophile (Nuc) attacks the substrate (R-LG) forming a new bond
• the leaving group (LG) departs with an electron pair, giving the main product (R-Nuc)
• The nucleophile may be electrically neutral or negatively charged
• the substrate is typically neutral or positively charged
• E.g. hydrolysis of an alkyl bromide

In IBDP organic chemistry,

• Two main mechanisms(SN1/SN2) at work in a nucleophilic substitution, both competing with each other
• S stands for chemical substitution, N stands for nucleophilic and the number represents the kinetic order of the reaction

• First order, two-step reaction
• (1) the removal of the leaving group creates a positively charged carbon(electrophile)
• (2) nucleophile approaches to the carbocation and bond by donating the electrons
• usually happen when the carbocation is not easily accessible (surrounded with bulky group)

• in above example, nucleophile = Cl ion, tertiary carbon ---> SN1

•  Second order, simultaneous reaction
• the removal of the leaving group and approaching of nucelophile take place at the same time
• usually occur where the central carbon atom is easily accessible to the nucleophile (no bulky group surrounding the carbocation)

• in above example, nucleophile = ammonia with lone pair electrons, primary carbon ---> favors SN2

(1) Electrophile

• leaving group is attached to methyl/primary carbon ---> favors SN2 (less substituted C)
• leaving group is attached to tertiary/allylic/bezylic carbon ---> favors SN1 (more substituted C/ bulky group attached)

(2) Nucleophile

• more negatively charged nucleophile ---> SN2
• weak nucleophile like H2O, alcohol ---> SN1

• Reactivity: Acid chlorides > acid anhydrides > esters > amides
• The more stable the leaving group, the easier it leaves
• The greater the electrophilic strength, the more reactive the acid derivative

• the more stable the ions formed as leaving group, the faster the nucleophilic substitution
• More electronegative atoms can stabilise a charge better as they tend to draw electrons towards themselves (i.e. Cl > O > N > C)
• A -ve charge can be stabilised or destabilised by the inductive effects of neighboring groups.
• A -ve charge can be stabilised by resonance effects

In IBDP organic chemistry,

• Nucleophilic substitution reactions are possible with carboxylic acid derivatives
• Reactivity: Acid chlorides > acid anhydrides > esters > amides
• Reactivity depends on (1) the stability of the leaving group (2) electrophilic strength of the carbonyl group
• IMPOSSIBLE to make a reactive acid derivative from a less reactive one
• Primary alkyl halides ---> SN2 mechanism
• Tertiary Alkyl Halides ---> SN1 mechanism
• Secondary Alkyl halides --->  SN1 or SN2 mechanisms
• if the alkyl groups are both small ---> SN2
• if the alkyl groups are both large ---> SN1

This is the end of this topic.