Zadanie 2b
Wzorowane na: Chemistry HL Paper 2, November 2024, pytanie 2(b). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Darmowe konto pozwoli wrócić do niego później.
Polecenie
2-Methylbut-1-ene reacts with hydrogen bromide, HBr, to give two structural isomers of C5H11Br.
(i) Deduce which of the two isomers is formed in the larger amount and explain your choice.
(ii) Using curly arrows to show the movement of electron pairs, explain the mechanism by which the major product forms.
(iii) One of the two isomers is chiral. Name an instrument that can distinguish between its two enantiomers.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) 2-bromo-2-methylbutane / CH3CBr(CH3)CH2CH3 ✔
the intermediate tertiary carbocation is better stabilized than the primary one
OR
three electron-releasing alkyl groups on the positively charged carbon «positive inductive effect» ✔
(ii) curly arrow from the C=C double bond to the H atom of HBr together with a curly arrow showing the H–Br bond breaking heterolytically ✔
representation of the tertiary carbocation, (CH3)2C+CH2CH3 ✔
curly arrow from a lone pair on the bromide ion, Br−, to the positively charged carbon atom ✔
(iii) polarimeter ✔
Schemat punktowania
(i) M1: correct isomer, as name or structure; M2: carbocation stability. Markovnikov's rule on its own is not an explanation and does not earn M2. (ii) M1: first curly arrow from the double bond together with the curly arrow for heterolytic fission of H–Br; M2: carbocation; M3: curly arrow from the bromide ion to the carbocation. If the isomer named in (i) is the wrong one, mark a mechanism that leads to it on its merits. (iii) [1].
Komentarz
The chiral isomer is the minor product, 1-bromo-2-methylbutane; the major product, 2-bromo-2-methylbutane, is not chiral.