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R3.4.13 – Electrophilic substitution of benzene

📌 Resonance

• Benzene is highly unsaturated but does not react like other alkenes (or unsaturated hydrocarbons)
• Due to it’s high stability and ring structure, benzene undergoes substitution rather than addition (as is the case with most double bond compounds)

📌 Electrophilic substitution

• The delocalised cloud of electrons attracts electrophilic species
• The hydrogen atom on one of the six carbon atoms can be substituted by the electrophile
• When an electron pair from the benzene is attracted to the electrophile, it causes a disruption in the delocalised pi bond system
• Following this, an intermediate compound is formed where both the leaving hydrogen and the electrophile are bonded to the benzene ring. This is represented by a dotted semicircle inside the ring compound
• After this, the hydrogen leaves the compound and the electrophile is substituted in its place

📌 Nitration of benzene

• 0One important reaction involving the electrophilic substitution of benzene is the nitration reaction
• This involves the substitution of NO₂⁺ into the ring compound
• First, the nitronium ion is generated using the nitrating mixture of concentrated sulphuric acid and concentrated nitric acid at 50°C
• Once the nitronium ion is generated, it undergoes the same reaction mechanism as seen above to form nitrobenzene

R3.4.12 – Addition of hydrogen halides to asymmetrical alkenes

📌 Asymmetrical alkenes

• Asymmetrical alkenes are ones in which more than one type of product can be formed
• To determine which product is more stable (ie likely to be produced), the number of positive induvctive effects acting on the central carbon atom must be determined
• The greater the inductive effect, the more stable the product is

📌 Markovnikov’s rule

• Markovnikov’s rule states that the more electropositive part of the reacting species bonds to the least highly substituted carbon atom in the alkene
• Using this rule, we can determine which product will be more likely to be produced, and therefore determine the reaction mechanism

R3.4.11 – Electrophilic addition reaction mechanisms (HL)

📌 Breaking of the pi bond

• Addition reactions involving alkenes require the breaking of the pi bond to create two single (sigma) bonds
• From information learnt in S2, we know that the pi bond is electron dense
• This electron dense area attracts electrophiles so that alkenes can undergo addition reactions

📌 Reaction mechanism

• Addition reactions involve a 2-step mechanism
• First, the electrophile undergoes heterolytic fission, following which the partially positive ion is attracted to the electron dense area of the pi bond
• This then creates an intermediate carbocation and a negative ion
• The negative ion is then attracted to the positive charge on the central carbon of the carbocation
• This reaction mechanism works for both halogens and hydrogen halides
• In the case of hydrogen halides, the hydrogen atom will always attach first, followed by the halogen

📌 Addition of water

• The addition of water can only occur in the presence of a strong catalyst as water is a weak electrophile
• First, the formation of a H₃O⁺ ion which acts as a catalyst (any strong acid)
• The heterolytic fission of the H₃O⁺results in the formation of a carbocation intermediate and a single water molecule
• Following this, the water molecule acts as a nucleophile and bonds to the intermediate carbocation

R3.4.9 & R3.4.10 – S_N1 and S_N2 nucleophilic substitution mechanisms (HL)

📌 Types of halogenoalkanes

• Halogeonalkanes can be divided into 3 subgroups : primary, secondary and tertiary. The iB syllabus focuses on the reaction mechanisms of primary and tertiary halogeonalkanes
• Primary : Primary halogeonalkanes have at least 2 hydrogens attached to the carbon with the C-X bond (X is a halogen). These compounds undergo S_N2 reactions which are substitution reactions involving nucleophiles (S is for substitution, N is for nucelophilic)
• Tertiary : Tertiary halogenoalkanes have three alkyl groups attached to the carbon with the C-X bond. These compounds undergo S_N1 reactions

📌 Understadning S_N2 reactions

• In such reactions, the C-X bonds breaks heterolytically, causing the halogen ion to be released. Additionally, the carbon takes on a slightly positive charge which causes the nucleophile to be attracted to it.
• This then forms an unstable intermediate state wherein both the nucleophile and the halogen are attached to the central carbon. The C-X bond then fully breaks and the halogen ion is released /
• Because the reactions depends on the concentrations of both the nucleophile and the halogenoalkane, S_N2 reactions are bimolecular (hence the 2), meaning that they are second order overall
• Note that the nucleophile attaches itself on the opposite side from the halogen which causes an inversion of arrangement of atoms/groups around the central carbon. This is why S_N2 reactions are considered to be stereospecific.

📌 Understadning S_N1 reactions

• In such reactions, the alkyl groups around the central carbon cause steric hinderance
• Unlike in an S_N1 reactions, when the C-X bond breaks heterolytically here, the halogen ion becomes the leaving group immediately and an intermediate cation is formed
• This intermediate is known as a carbocation and becomes easily attracted to the nucleophile
• Such reactions depend only on the concentration of the halogenoalkane, making it unimolecular (hence the 1)
• These reactions are no stereospecific and they demonstrate the idea of ‘positive inductive effect’ due to the 3 alkyl groups which stabilise the compound

📌 Halogens

• The quality of the leaving group depends on the strength of the C-X bond
• Greater bond strength means less energetically favourable for the bond to be broken
• The leaving groups can be ranked as the following : I^– > Br^– > Cl^– > F^– with iodoalkanes being the best and fluoroalkanes being the worst

R3.4.8 – Complex ions (HL)

📌 Chemistry of transition elements

• Transition metal ions act as Lewis acids with ligands acting as Lewis bases
• When coordinate bonding between transition metals and ligands occurs, complex ions are formed
• Ligands can be neutral, but they must always contain a lone pair of electrons (since Lewis bases are nucleophiles)
• Transition metals usually bond to several ligands and the number of coordinate bonds a metal ion can form is known as the ‘coordination number’

📌 Charge on ions

• The charge on a complex ion depends on three factors : the charge on the central metal, the charge on the ligands and the coordination number
• The charge on the complex ion can be found by adding the charge of the central metal ion to the charge of each ligand present

R3.4.6 & R3.4.7 – Lewis acids and bases (HL)

📌 Defining acids and bases

• A Lewis acid is defined as a species that can accept a lone pair of electrons
• A Lewis base is defined as a species that can donate a lone pair of electrons
• Unlike the Brønsted-Lowry theory covered earlier, this theory focuses on lone pairs of electrons
• By these definitions we can understand that Lewis acids are electrophiles while lewis bases are nucleophiles

R3.4.5 – Electrophilic addition of alkenes

📌 Alkenes

• Alkenes are unsaturated hydrocarbons with a C=C double bond
• The double bond is electron dense which enables it to undergo reactions with electrophiles
• The pi bond in the double bond is selectively broken to form two new single bonds. This enables addition reactions
• Addition reactions only have a single product

📌 Addition of water

• Adding water across the double bond in an alkene is known as hydration
• Highly concentrated sulphuric acid is used as a catalyst for this reaction due to water’s weak properties as an electrophile
• The product of hydration of an alkene is an alcohol with the same number of carbons (eg. ethene becomes ethanol)

📌 Addition of halogens

• Addition of halogens produces dihalogeno compounds
• The pi bond breaks and forms two new single bonds which each bond to one halogen atom

📌 Addition of halogen halides

• Addition of hydrogen halides produces halogenoalkanes
• All hydrogen halides can undergo addition reactions with alkenes but the order of the halogens varies
• HI reacts most readily followed by HBr then HCl

R3.4.4 – Electrophiles

📌 What are electrophiles?

• Electrophiles are electron-deficient species
• Electrophiles acccept electron pairs from other species (nucleophiles) to form a coordinate bond
• Electrophiles can be neutral or positively charged

R3.4.3 – Heterolytic fission

📌Products of fission

• Heterolytic fission produces oppositely charged ions
• When a covalent bond breaks and one product gains both electrons, one n egatively charged ion and one positively charged ion is produced
• Again, a double-headed arrow is used to show the movement of the electron pair

R3.4.2 – Nucleophilic substitution reactions

📌 Halogenoalkanes

• Halogenoalkanes are polar, causing the carbon to be slightly electron deficient (positive) and the halogen to be slightly electron rich (negative)
• Since nucleophiles are electron rich, they are attracted to the partially positive carbon in the halogenoalkane
• The nucleophile is then substituted in the position of the halogen
• The halogen then becomes the ‘leaving group’
• Halogens are good leaving groups because they form relatively weak bonds
• When the nucleophile is a neutral species (like water) the initial product is positively charged and deprotonates to form a neutral compound