Formula sheet: Hydrocarbons

Hydrocarbons18 min readLesson 1 of 7

1.1 What this sheet collects A hydrocarbon contains only carbon and hydrogen, and the chapter splits them into saturated alkanes, unsaturated alkenes and alkynes, and aromatic arenes. This sheet keeps the formulas and reagents together, because most examination questions ask either for a product of a named reaction or for the class and formula of a compound. Alkanes are saturated and open chain, with the general formula CnH2n+2, and they react mainly by substitution. Alkenes contain one carbon-carbon double bond and have the formula CnH2n, and they react by addition at the double bond. Alkynes contain one carbon-carbon triple bond and have the formula CnH2n-2, and they also react by addition, while terminal alkynes are weakly acidic. Aromatic hydrocarbons follow the Huckel rule, and benzene is the parent arene.

C_{n} H_{2n + 2} (alkane), C_{n} H_{2n} (alkene), C_{n} H_{2n - 2} (alkyne or cycloalkane)

General formulas of alkanes, alkenes and alkynes

Complete combustion of a hydrocarbon gives carbon dioxide and water, and the oxygen needed follows directly from the formula. These equations are the fastest way to solve the numerical questions on combustion.

C_{n} H_{2n + 2} + \frac{3 n + 1}{2} O_{2} \to n C O_{2} + (n + 1) H_{2} O

Complete combustion of an alkane

C_{n} H_{2n} + \frac{3 n}{2} O_{2} \to n C O_{2} + n H_{2} O, C_{n} H_{2n - 2} + \frac{3 n - 1}{2} O_{2} \to n C O_{2} + (n - 1) H_{2} O

Complete combustion of an alkene and an alkyne

Incomplete combustion gives carbon monoxide or carbon black instead of carbon dioxide, and it is the reason a yellow sooty flame appears when the air supply is limited. Addition of hydrogen halides to unsymmetrical alkenes follows the Markovnikov rule, while the peroxide effect reverses the orientation for HBr only.

C H_{3} - CH = C H_{2} + HBr \to C H_{3} - CHBr - C H_{3} (major, Markovnikov)

Markovnikov addition of HBr to propene

Ozonolysis cleaves the double bond and gives carbonyl compounds, and it is used to locate the position of the double bond. Cold dilute potassium permanganate gives a vicinal glycol and its purple colour disappears, which is the test for unsaturation.

R_{2} C = C R_{2} + O_{3} Zn/ H_{2} O R_{2} C = O + O = C R_{2}, R_{2} C = C R_{2} + KMn O_{4} (cold, dilute) \to vicinal glycol

Ozonolysis and the Baeyer oxidation

Aromaticity needs a planar ring, complete conjugation and 4n plus 2 pi electrons in the ring. Benzene has six delocalised pi electrons, and it reacts mainly by electrophilic substitution, which keeps that aromatic ring intact.

Huckel rule: (4 n + 2) π electrons, n = 0, 1, 2, \dots

Huckel rule for aromaticity

Exam

Keep the reagents with the reactions: cold dilute alkaline permanganate for the Baeyer test and glycols, acidic permanganate or dichromate for cleavage, ozone followed by zinc and water for ozonolysis, hydrogen with nickel or platinum for complete reduction, hydrogen with Lindlar catalyst for a cis alkene, and sodium in liquid ammonia for a trans alkene. For benzene substitution, remember the catalyst for each reaction: concentrated sulphuric acid with nitric acid for nitration, sulphuric acid for sulphonation, and aluminium chloride or ferric bromide with the halogen for halogenation.

in hinglish

Formula sheet ke main points: alkane CnH2n+2, alkene CnH2n aur alkyne CnH2n-2; aromatic compounds Hückel rule (4n+2 π electrons) follow karte hain. Complete combustion mein alkane ke liye (3n+1)/2 O2, alkene ke liye 3n/2 O2 aur alkyne ke liye (3n-1)/2 O2 lagta hai. Markovnikov rule ke hisaab se HBr unsymmetrical alkene par zyada stable carbocation wali side add hota hai, aur peroxide ki presence mein sirf HBr ka addition anti-Markovnikov ho jata hai. Ozonolysis double bond todkar aldehyde/ketone deta hai, aur cold dilute KMnO4 (Baeyer reagent) ka purple colour Khatam hona unsaturation ka test hai. Benzene planar ring mein 6 π electrons rakhta hai aur electrophilic substitution karta hai; nitration ke liye conc HNO3 + H2SO4, sulphonation ke liye H2SO4, halogenation ke liye FeBr3/AlCl3 aur Friedel-Crafts ke liye AlCl3 yaad rakho.