15. Halogen Compounds Flashcards
(11 cards)
What are the three ways halogenoalkanes can be produced?
- The free radical substitution of alkanes
- The electrophilic addition of X₂ or HX to an alkene
- The nucleophilic substitution of an alcohol with various compounds
What reagents react with alcohols to form halogenoalkanes via nucleophilic substitution and what else will be produced in each reaction?
- HX (g) (produces H₂O)
- KX with concentrated H₂SO₄ or concentrated H₃PO₄ to form HX (produces H₂O)
- Heated PCl₃ (produces phosphorous acid - H₃PO₃)
- PCl₅ (produces HCl and POCl₃)
- SOCl₂ (produces HCl and SO₂)
Each reaction produces the expected halogenoalkane as well as the additional products listed
How are halogenoalkanes classified?
- A primary halogenoalkane has the halogen bonded to a carbon that is bonded to one (or no) alkyl groups
- A secondary halogenoalkane has the halogen bonded to a carbon that is bonded to two alkyl groups
- A tertiary halogenoalkane has the halogen bonded to a carbon that is bonded to three alkyl groups
What is the reaction of NaOH (aq) with bromoethane?
The mechanisms are discussed in another card
- Aqueous sodium hydroxide and bromoethane are reacted under heat
- The OH⁻, behaving as a nucleophile, replaces the halogen
- Ethanol and Br⁻ are produced
- This is a nucleophilic substitution reaction
If NaOH is ethanolic, the elimination reaction detailed in the previous deck will occur and produce an alkene
What is the reaction of ethanolic KCN with bromoethane?
- Ethanolic KCN is heated under reflux with bromoethane, generating the CN⁻ in situ
- The CN⁻ behaves as a nucleophile, replacing the halogen
- Propanenitrile and Br⁻ are produced
- This is a nucleophilic substitution reaction
This reaction is valuable as it can be used to lengthen carbon chains
What is the reaction of ethanolic NH₃ with bromoethane?
- Ethanolic NH₃ is reacted with bromoethane under heat and pressure
- The NH₃ behaves as a nucleophile, replacing the halogen
- Ethylamine and Br⁻ are produced
- This is a nucleophilic substitution reaction
Ammonia should be in excess as ethylamine can react with bromoethane
What is the reaction of ethanolic aqueous silver nitrate with halogenoalkanes and how can it be used to identify the halogen?
- The halogenoalkanes undergo hydrolysis in aqueous solution, accepting OH⁻ and forming alcohols in a nucleophilic substitution reaction
- The freed X⁻ ion then reacts with silver ions in solution to form an AgX precipitate
- AgCl is white, AgBr is cream and AgI is yellow
- The rate of hydrolysis is another identifying factor; C-I bonds are the weakest while C-Cl bonds are the strongest
- This means iodoalkanes undergo nuclephilic substitution the fastest so AgI will be produced at the fastest rate in this reaction; the opposite is true for chloroalkanes
The nucleophilic substitution of halogenoalkanes in water is slower than in aqueous NaOH as the OH⁻ in NaOH has a formal negative charger rather than a partial negative charge
What are the two mechanisms of nucleophilic substitution called and which halogenoalkanes undergo each one?
- SN1, undergone by tertiary halogenoalkanes
- SN2, undergone by primary halogenoalkanes
- Secondary halogenoalkanes can undergo either
What is the SN1 mechanism?
Exemplified by the reaction of NaOH (aq) and 2-bromo-2-methyl propane
- There is a strong dipole in the C-Br bond
- It breaks heterolytically (rate-determining step), producing an Br⁻ ion and a tertiary carbocation
- This carbocation is attacked by an OH⁻ (nucleophile), which donates a lone pair and forms a bond
- This results in the formation of a tertiary alcohol (2-methyl-2-propanol) and Br⁻ (which can bond to Na⁺, but this does not need to be shown)
What is the SN2 mechanism?
Exemplified by the reaction of NaOH (aq) and bromoethane
- An OH⁻ nucleophile donates a pair of electrons to the carbon atom bonded to the Br
- Simultaneously, the C-Br bond undergoes heterolytic fission, with both electrons going to Br
- The halogen leaves the compound while the OH⁻ enters, forming ethanol and Br⁻
- The Br and OH are often shown bonded to the same carbon at the same time, though the bonds are represented with dotted lines as this intermediate is very transient
- Unlike SN1, as this is a single-step process, the rate is determined by the concentration of both the halogenoalkane and the nucleophile
- Remember that all the nucleophilic substitution reactions discussed in this deck occur through either SN1 or SN2 depending on the classification of the halegenoalkane
What is the explanation for why some halogenoalkanes undergo SN1 and some undergo SN2?
- Tertiary halegenoalkanes produce tertiary carbocations after the halogen leaves
- These carbocations are stable as three alkyl groups push electron density toward them (inductive effect)
- Thus, tertiary halegenoalkanes can undergo SN1 (a two-step reaction), as the carbocation intermediate is stable enough to form
- For primary halegenoalkanes, the potential carbocation is too unstable to form as it would have only one nearby alkyl group, so the whole substitution must occur in a single step (SN2)
