Hydrocarbons Formula Sheet

Last-minute revision sheet for the entire Hydrocarbons chapter. Organic chemistry is reaction-driven, so this is built around general formulas, named reactions, reactivity/stability orders, and the high-yield facts you must recall in the exam hall, rather than numeric formulas.

General Formulas and Bond Data

Family	General formula	Bond	Length	Bond energy
Alkanes	$\text{C}_n\text{H}_{2n+2}$	C-C (single)	154 pm	347 kJ/mol
Alkenes	$\text{C}_n\text{H}_{2n}$	C=C (double)	134 pm	611 kJ/mol
Alkynes	$\text{C}_n\text{H}_{2n-2}$	C≡C (triple)	120 pm	839 kJ/mol

$$\boxed{\text{Alkane: } C_nH_{2n+2} \qquad \text{Alkene: } C_nH_{2n} \qquad \text{Alkyne: } C_nH_{2n-2}}$$

• $\pi$ bond energy in C=C $= 611 - 347 = 264$ kJ/mol.
• Bond strength: C≡C > C=C > C-C; bond length: C≡C < C=C < C-C.

Degree of Unsaturation (DBE)

$$\boxed{\text{DBE} = \frac{2C + 2 - H}{2}}$$

For benzene (C₆H₆): $\text{DBE} = \dfrac{2(6)+2-6}{2} = 4$.

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Alkanes

Preparation

Method	Reaction	Key note
Wurtz	$2RX + 2Na \xrightarrow{\text{dry ether}} R\text{-}R + 2NaX$	Symmetrical alkanes only
Kolbe electrolysis	$2RCOO^-Na^+ \xrightarrow{\text{electrolysis}} R\text{-}R + 2CO_2 + 2Na^+ + 2e^-$	Decarboxylation at anode
Reduction of R-X	$RX + 2[H] \xrightarrow{Zn/HCl} R\text{-}H + HX$	Also LiAlH₄ (milder)
Clemmensen	$R\text{-}CO\text{-}R' \xrightarrow{Zn\text{-}Hg/HCl} R\text{-}CH_2\text{-}R'$	Acidic medium
Wolff-Kishner	$R\text{-}CO\text{-}R' \xrightarrow{NH_2NH_2,\,KOH} R\text{-}CH_2\text{-}R'$	Basic medium
Sodalime decarboxylation	$RCOONa \xrightarrow{CaO+NaOH,\,\Delta} R\text{-}H + Na_2CO_3$	Loses one carbon

$$\boxed{2RX + 2Na \xrightarrow{\text{dry ether}} R\text{-}R + 2NaX \quad (\text{Wurtz})}$$

Combustion

$$\boxed{C_nH_{2n+2} + \frac{3n+1}{2}\,O_2 \rightarrow n\,CO_2 + (n+1)\,H_2O + \text{Heat}}$$

For methane: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O,\ \Delta H = -890$ kJ/mol.

Free-Radical Halogenation

$$\boxed{R\text{-}H + X_2 \xrightarrow{h\nu\ \text{or}\ \Delta} R\text{-}X + HX}$$

• Initiation: $Cl_2 \xrightarrow{h\nu} 2Cl^\bullet$ (homolysis, Cl-Cl = 242 kJ/mol)
• Propagation: $Cl^\bullet + CH_4 \rightarrow CH_3^\bullet + HCl$; then $CH_3^\bullet + Cl_2 \rightarrow CH_3Cl + Cl^\bullet$
• Termination: radical-radical recombination

H-atom reactivity (radical stability):

$$\boxed{3^\circ > 2^\circ > 1^\circ > CH_4}$$

Bond dissociation energies: CH₃-H 435, (CH₃)₂CH-H 410, (CH₃)₃C-H 390 kJ/mol (lower BDE = more reactive).

Halogen reactivity:

$$\boxed{F_2 > Cl_2 > Br_2 > I_2}$$

(F₂ explosive/non-selective; Br₂ slow but highly selective; I₂ endothermic, does not proceed.)

Product ratio rule:

$$\boxed{\text{Product fraction} = \frac{(\text{No. of H of that type}) \times (\text{relative reactivity})}{\sum (\text{No. of H}) \times (\text{relative reactivity})}}$$

Relative reactivity per H (chlorination): $1^\circ : 2^\circ : 3^\circ = 1 : 4 : 5.5$.

Conformations (Newman Projections)

System	Stability order	Key energies
Ethane	Staggered (60°) > Eclipsed (0°)	Barrier = 12.5 kJ/mol
Butane (C2-C3)	Anti > Gauche > Eclipsed > Fully eclipsed	Gauche +3.8; eclipsed +16; fully eclipsed +19 kJ/mol

$$\boxed{\text{Anti} > \text{Gauche} > \text{Eclipsed} > \text{Fully eclipsed}}$$

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Alkenes

Preparation

Method	Reaction	Rule / note
Dehydrohalogenation	$R\text{-}CH_2\text{-}CH_2X \xrightarrow{\text{alc. KOH},\,\Delta} R\text{-}CH=CH_2 + HX$	Saytzeff (E2)
Dehydration of alcohols	$R\text{-}CH_2\text{-}CH_2OH \xrightarrow{\text{conc. }H_2SO_4,\,443\,K} R\text{-}CH=CH_2 + H_2O$	E1; watch carbocation rearrangement
Dehalogenation	$R\text{-}CHX\text{-}CHX\text{-}R + Zn \rightarrow R\text{-}CH=CH\text{-}R + ZnX_2$	Also 2KI/acetone
From alkynes (Lindlar)	$R\text{-}C{\equiv}C\text{-}R + H_2 \xrightarrow{Pd/BaSO_4} cis\text{-alkene}$	Syn addition
From alkynes (Na/NH₃)	$R\text{-}C{\equiv}C\text{-}R + 2Na/\text{liq. }NH_3 \rightarrow trans\text{-alkene}$	Anti addition

Saytzeff’s rule: more substituted (more stable) alkene is the major product.

Ease of dehydration: $3^\circ > 2^\circ > 1^\circ$ alcohols (carbocation stability).

Stability of Alkenes

$$\boxed{\text{Tetra} > \text{Tri} > \text{Di} > \text{Mono-substituted} > \text{Ethylene}}$$

For equal substitution: trans > cis. Stability $\propto \dfrac{1}{\text{heat of hydrogenation}}$ (lower $\Delta H$ released = more stable).

Electrophilic Addition Reactions

Reaction	Reagent	Product	Markovnikov? / stereochem
Hydrogenation	H₂ / Pt, Pd, Ni	Alkane	Syn addition
Halogenation	X₂ / CCl₄	Vicinal dihalide	Anti (bromonium ion); decolourises Br₂
Hydrohalogenation	HX	Alkyl halide	Markovnikov
Peroxide effect	HBr / peroxide	Alkyl halide	Anti-Markovnikov (HBr only)
Acid hydration	H₂SO₄ / H₂O	Alcohol	Markovnikov
Oxymercuration	Hg(OAc)₂ then NaBH₄	Alcohol	Markovnikov, no rearrangement
Hydroboration-oxidation	B₂H₆ then H₂O₂/OH⁻	Alcohol	Anti-Markovnikov, syn
Ozonolysis	O₃ then Zn/H₂O	Aldehydes / ketones	Cleavage
KMnO₄ (cold, dilute)	KMnO₄	vic-Diol	Baeyer’s test
KMnO₄ (hot, conc.)	KMnO₄, $\Delta$	Carboxylic acids	Oxidative cleavage

Markovnikov’s rule: H adds to the carbon already bearing more H; halogen / positive charge goes to the more substituted carbon (more stable carbocation).

$$\boxed{CH_3\text{-}CH=CH_2 + HBr \rightarrow CH_3\text{-}CHBr\text{-}CH_3}$$

HX reactivity: $HI > HBr > HCl > HF$ (weakest bond / best leaving group reacts fastest).

Peroxide effect (Kharasch): works with HBr only — H-Cl too strong (431 kJ/mol), H-I too weak (299 kJ/mol), H-Br is “just right” (366 kJ/mol).

Ozonolysis & Stereochemistry of Halogenation

• Reductive workup (Zn/H₂O) → aldehydes/ketones; oxidative workup (H₂O₂) → carboxylic acids.
• Br₂ addition is anti: cis-2-butene → meso-2,3-dibromobutane; trans-2-butene → racemic mixture.

Polymerization

$$\boxed{n\,CH_2{=}CH_2 \xrightarrow{\text{catalyst}} (\text{-}CH_2\text{-}CH_2\text{-})_n}$$

Examples: polyethylene, polypropylene, PVC (from vinyl chloride), polystyrene.

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Alkynes

Preparation

Method	Reaction
Calcium carbide	$CaC_2 + 2H_2O \rightarrow HC{\equiv}CH + Ca(OH)_2$
Carbide synthesis	$CaO + 3C \xrightarrow{2000^\circ C} CaC_2 + CO$
Vicinal dihalide	$R\text{-}CHX\text{-}CHX\text{-}R + 2KOH\,(\text{alc.}) \xrightarrow{\Delta} R\text{-}C{\equiv}C\text{-}R + 2KX + 2H_2O$
Geminal dihalide	$R\text{-}CHX_2\text{-}CH_3 + 2KOH\,(\text{alc.}) \xrightarrow{\Delta} R\text{-}C{\equiv}CH + 2KX + 2H_2O$
Alkylation	$HC{\equiv}C^-Na^+ + R\text{-}X \rightarrow R\text{-}C{\equiv}CH + NaX$

Acidic Character (Terminal Alkynes)

$$\boxed{HC{\equiv}CH > H_2C{=}CH_2 > H_3C\text{-}CH_3 \quad (\text{acidity})}$$

Compound	Hybridisation	s-character	pKa
Ethyne	sp	50%	25
Ethene	sp²	33%	44
Ethane	sp³	25%	50

More s-character → more electronegative C → stabilises the carbanion → more acidic.

Acetylide formation:

$$\boxed{R\text{-}C{\equiv}C\text{-}H + NaNH_2 \rightarrow R\text{-}C{\equiv}C^-Na^+ + NH_3}$$

• Use NaNH₂, not NaOH: pKa(NH₃)=35 > pKa(alkyne)=25 > pKa(H₂O)=15.7, so NaOH is too weak.
• Also reacts with Na metal ($+\tfrac12 H_2$) and Grignard reagents ($+CH_4$).

Test for terminal alkynes:

$$\boxed{R\text{-}C{\equiv}CH + AgNO_3 + NH_3 \rightarrow R\text{-}C{\equiv}C\text{-}Ag\downarrow + NH_4NO_3}$$

White/grey ppt with ammoniacal AgNO₃; red-brown ppt with ammoniacal Cu₂Cl₂. Internal alkynes give no precipitate.

Addition Reactions (two moles can add)

Reagent	Product	Note
H₂ / Pt (excess)	Alkane	Complete reduction
H₂ / Lindlar (Pd-BaSO₄, quinoline)	cis-Alkene	Syn
Na / liq. NH₃	trans-Alkene	Anti
X₂ (1 mol) → (2 mol)	Dihaloalkene → tetrahaloalkane	$R\text{-}CX{=}CX\text{-}R \to R\text{-}CX_2\text{-}CX_2\text{-}R$
HX (excess)	Geminal dihalide	Markovnikov each step → both X on same C
H₂O / H₂SO₄, HgSO₄	Ketone (or acetaldehyde from ethyne)	Via enol, Markovnikov
(sia-BH)₂ then H₂O₂/OH⁻	Aldehyde	Anti-Markovnikov hydration

$$\boxed{R\text{-}C{\equiv}CH \xrightarrow{H_2SO_4,\ HgSO_4} R\text{-}CO\text{-}CH_3 \quad (\text{via enol, keto-enol tautomerism})}$$

Special case: $HC{\equiv}CH \xrightarrow{H_2SO_4,\,HgSO_4} CH_3CHO$ (acetaldehyde). Keto : enol $\approx 10^6 : 1$.

Excess HX gives geminal (not vicinal) dihalides: propyne + 2HBr → CH₃-CBr₂-CH₃.

Polymerisation of Ethyne

$$\boxed{3\,HC{\equiv}CH \xrightarrow{\text{Red-hot Fe tube, 873 K}} C_6H_6 \quad (\text{benzene, cyclic trimerisation})}$$

PVC route: $HC{\equiv}CH + HCl \xrightarrow{HgCl_2,\,333\,K} CH_2{=}CHCl \xrightarrow{\text{peroxide}} (\text{-}CH_2\text{-}CHCl\text{-})_n$.

Distinguishing Tests

Test	Alkane	Alkene	Terminal alkyne	Internal alkyne
Br₂/CCl₄	No reaction	Decolourises	Decolourises	Decolourises
KMnO₄	No reaction	Decolourises	Decolourises	Decolourises
Ammoniacal AgNO₃	No ppt	No ppt	White ppt	No ppt
NaNH₂	No reaction	No reaction	Forms acetylide	No reaction

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Benzene and Aromaticity

Structure

• C₆H₆, planar regular hexagon; all C-C bonds equivalent at 139 pm, bond order 1.5.
• Six delocalised $\pi$ electrons; resonance energy = 152 kJ/mol (extra stability vs hypothetical cyclohexatriene).

Hückel’s Rule of Aromaticity

A species is aromatic if it is cyclic, planar, fully conjugated, and has:

$$\boxed{(4n+2)\ \pi \text{ electrons}, \quad n = 0, 1, 2, 3, \dots}$$

• Aromatic counts: 2, 6, 10, 14… ($\pi$ e⁻); 6 is most common (n = 1).
• Anti-aromatic (planar): $4n$ → 4, 8, 12… $\pi$ e⁻ (highly unstable).
• Examples: benzene (6), naphthalene (10), pyridine (6), furan (6), cyclopentadienyl anion (6), tropylium cation (6) are aromatic; cyclobutadiene (4) anti-aromatic; cyclooctatetraene non-planar → non-aromatic.

Electrophilic Aromatic Substitution (EAS)

$$\boxed{C_6H_6 + E^+ \xrightarrow{\text{catalyst}} C_6H_5\text{-}E + H^+}$$

Mechanism: (1) E⁺ attacks ring → arenium ion (Wheland intermediate), slow/RDS, aromaticity lost temporarily; (2) loss of H⁺ restores aromaticity (fast). Benzene undergoes substitution (not addition) to retain 152 kJ/mol of aromatic stabilisation.

Reaction	Reagent	Electrophile	Product
Halogenation	X₂ / FeX₃ (Lewis acid)	X⁺	C₆H₅-X
Nitration	HNO₃ / conc. H₂SO₄	NO₂⁺	C₆H₅-NO₂
Sulfonation	Fuming H₂SO₄ (oleum)	SO₃ / HSO₃⁺	C₆H₅-SO₃H (reversible!)
Friedel-Crafts alkylation	R-Cl / AlCl₃	R⁺	C₆H₅-R
Friedel-Crafts acylation	RCOCl / AlCl₃	RC≡O⁺ (acylium)	C₆H₅-CO-R

• Nitronium generation: $HNO_3 + 2H_2SO_4 \rightarrow NO_2^+ + H_3O^+ + 2HSO_4^-$.
• Sulfonation is reversible: $C_6H_5SO_3H + H_2O \xrightarrow{\text{heat, steam}} C_6H_6 + H_2SO_4$ (useful for blocking a position).

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Directive Effects in EAS

Two independent properties: orientation (o/p vs m) and reactivity (activating vs deactivating).

Master Classification Table

Substituent	Electronic effect	Direction	Reactivity vs benzene
-O⁻, -OH, -NH₂	+R ≫ -I	ortho/para	$10^3$-$10^6$× faster
-OR, -NHCOR (-NHCOCH₃, -OCOCH₃)	+R > -I	ortho/para	10-$10^2$× faster
-R (alkyl: -CH₃, -C₂H₅)	+I only	ortho/para	2-25× faster
-F, -Cl, -Br, -I	-I > +R	ortho/para	$10^{-1}$-$10^{-3}$× slower
-CHO, -COR, -COOH	-I, -R	meta	$10^{-2}$-$10^{-4}$× slower
-CN, -SO₃H	-I, -R	meta	$10^{-4}$-$10^{-6}$× slower
-NO₂, -NR₃⁺	-I, -R	meta	$10^{-6}$-$10^{-8}$× slower

Reactivity Orders to Memorise

Activating o/p directors:

$$\boxed{\text{-}O^- > \text{-}OH > \text{-}OR > \text{-}NH_2 > \text{-}NHR > \text{-}NR_2}$$

Alkyl (+I) strength:

$$\boxed{\text{-}C(CH_3)_3 > \text{-}CH(CH_3)_2 > \text{-}CH_2CH_3 > \text{-}CH_3}$$

Halogen-substituted ring reactivity (all slower than benzene):

$$\boxed{C_6H_6 > C_6H_5F > C_6H_5Cl > C_6H_5Br > C_6H_5I}$$

Deactivating meta directors:

$$\boxed{\text{-}NR_3^+ > \text{-}NO_2 > \text{-}CN > \text{-}SO_3H > \text{-}COOH > \text{-}CHO > \text{-}COR}$$

The Golden Rules

Synthesis Strategy (order matters)

• Target with groups meta to each other → add the meta director first (e.g. nitrate, then halogenate for m-bromonitrobenzene).
• Target with groups ortho/para → add the o/p director first.
• Toluene nitration example ratio: ortho : meta : para ≈ 42 : trace : 58.
• Reactivity comparison: $\text{Nitrobenzene} < \text{Benzene} < \text{Toluene} < \text{Phenol}$.

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One-Glance Reactivity & Stability Orders

Quantity	Order
Radical / H-abstraction reactivity	$3^\circ > 2^\circ > 1^\circ > CH_4$
Halogen reactivity (alkanes)	$F_2 > Cl_2 > Br_2 > I_2$
Alkene stability	Tetra > Tri > Di > Mono > Ethylene; trans > cis
Alcohol ease of dehydration	$3^\circ > 2^\circ > 1^\circ$
HX addition reactivity	$HI > HBr > HCl > HF$
Acidity of hydrocarbons	alkyne > alkene > alkane
Conformer stability (butane)	Anti > Gauche > Eclipsed > Fully eclipsed
EAS reactivity (sample)	Phenol > Toluene > Benzene > Nitrobenzene

See Also

• Alkanes
• Alkenes
• Alkynes
• Benzene
• Directive Effects