Alkanes Flashcards
What has the general formula CnH2n?
Cycloalkanes - with only 1 ring
Alkenes
Phy. properties of alkanes (4)
- IMF - essentially np - relatively weak instantaneous dipole-induced dipole interactions
- BP & MP
- Increases with increasing no. of C - as the no. of C atoms in alkanes increases, the no. of e- per alkane molecule increases and the size of e- cloud increases. Hence, the ease of polarising the e- cloud increases, leading to increasing strength of instantaneous dipole-induced dipole attractive forces which are overcome at increasingly higher temp
- Decreases with increasing degree of branching
With branching, the molecule becomes more spherical and its surface area available for intermolecular interactions decreases. This reduces the extent of contact b/w neighbouring molecules, resulting in weaker instantaneous dipole-induced dipole forces that are overcome at a lower temp - solubility
Being essentially np, alkanes are soluble in np solvents such as benzene and insoluble in water and other highly polar solvents
Some l alkanes are used as np solvents - dissolve compounds of low polarity and do not dissolve compounds of high polarity - Density
Density increases with increasing molecular size of the alkanes but tend to lvl off at abt 0.8 gcm^-3
ALL LESS DENSE THAN WATER
Carbon & Hydrogen atoms being classified as primary (1°), secondary (2°), tertiary (3°) or quaternary (4°) - how it looks like
A 1°C atom is bonded to 1 other C atom …….
A 1°H atom is bonded to a C atom that is bonded to 1 other C atom
A 2°H atom is bonded to a C atom that is bonded to 2 other C atom
which alkanes are gas, liquid, gas?
Gas: methane, ethane, propane, butane
Liquid: Pentane, hexane, heptane, octane, nonane, decane, undercane…
Gas: … Icosane… ??
Reduction of alkenes (2)
Reduction of Alkenes -> alkanes
Reagent & Conditions: H2(g) & Ni & heat OR H2(g) & Pt/Pd
Reduction of other unsaturated hydrocarbons: Alkynes & benzene
Alkynes can be reduced to alkanes under suitable conditions
CH3C三CH (Propyne) + 2H2 —-(Pd)—–> CH3CH2CH3 (Propane)
Benzene can also be reduced to cyclohexane under suitable conditions
Benzene + 3H2 —-(Ni Catalyst, high temp, high pressure)—> cyclohexane
Reactions of alkanes with polar reagents: NaOH (aq) HCL (aq) Conc H2SO4 Acidified KMnO2 (aq) Air (O2) Bromine
NaOH (aq), HCL (aq), Conc H2SO4: No effect, 2 immiscible layers observed
Acidified KMnO2 (aq): No decolourisation of purple KMnO4 even on heating, 2 immiscible laters observed
Air (O2): No effect when cold. Burns when heated
Bromine: No effect in the dark. bromine decolourised in the presence of sunlight
The general reactivity of alkanes can be attributed to the following reasons: (2)
- NP
- they do not contain any region of high e- density (no C=C/C三C) & thus do not attract electrophilic reagents
- do not contain any e- deficient sites (no C=O bonds) to attract nucleophilic reagents - Alkanes have relatively strong C-C & C-H bonds which do not break under normal conditions
Reactions of Alkanes with np reagents (under right conditions)
(2)
Combustion & Halogenation (mechanism: free radical substitution)
Combustion of alkanes - exo? - complete n incomplete - general eqn for complete?
Burns readily in air or oxygen when ignited - very exo, impt source of fuel
Complete combustion produces CO2(g) & H2O (l) - non sooty flame
Combustion of a hydrocarbon: CxHy + (x+y/4) O2(g) -> xCO2(g) + y/2 H2O
Incomplete combustion leads to formation of CO (g) & C (as soot) in addition to CO2(g) & H2O(l) - sooty flame
CH4(g) + 3/2O2(G) -> CO (g) + 2H2O (l)
CH4(g) + O2(g) -> C(s) + 2H2O (l)
Defn of Free radical & characteristics eg electrically…
Free radical is an atom or grp of atoms that has an unpaired e-
- highly reactive & are intermediates in many rctns of alkanes
- electrically neutral as same no. of protons n e-
Stability of alkyl radicals & reason
Depends on the no. of alkyl grps ('R') attached
Methyl radical (0 R) < primary alkyl radical (1 R) < secondary alkyl radical (2 R) < tertiary alkyl radical (3 R) - most stable
Reason: The carbon with the unpaired e- is e- deficient. As alkyl groups are e- donating, the more alkyl grps attached to the carbon with the unpaired e-, the more stable the alkyl radical
Defn of homolytic fission of a covalent bond
occurs when the bond breaks in such a way that each of the atoms involved in forming the bond acquires one of the 2 bonding e-, thus forming free radials
(EQUAL SHARING OF E-)
Halogenation of an alkane involves … (defn)
Reagents:
Condition:
Mechanism:
Conducted in what state?
Halogenation of an alkane involves the substitution of 1 or more hydrogen atoms of the alkane by halogen atoms
Reagents: Cl2 or Br2
Condition: UV light or Heat
Mechanism: Free radical substitution
Conducted in liquid or gaseous state, not aq as water molecules will react with any radical present.
HCl in frs is called ___ not ____
Hydrogen chloride not hydrochloric acid cuz cant be aq state
Describing Free-radical substitution steps (8)
Name mechanism: Free radical substitution
Heading for 3 steps: Step 1: initiation, Step 2: propagation, Step 3: termination (basically combine all radicals)
Conditions for initiation: UV light/heat
Half arrows from the bond to halogen atoms in initiation step
Label propagation steps (a) & (b),
Then (a), (b), (a), (b)…
The termination steps must include a rctn which forms the required product (AT LEAST 3 products)
Placing of . on correct atoms in all steps
What to write when using additional propagation steps to generate prod
Propagation steps (a) & (b) give rise to many molecules of CH3CHBrCH3, which can undergo further bromination via propagation steps (c) & (d) to form CH3CHBrCH2Br
(c) the prod of (b) -> ……
(d) ….
Termination: all the radicals from (a) to (d)
Finding proportion of prod in f.r.s.
& factors affecting ^ (2)
- Ratio of diff types of H atoms in an alkane
- Stability of carbon radical intermediate
Ans: Based on probability factor and assuming all H atoms have the same reactivity, the approx molar ratio of G to H = 2:12 (secondary H atom: pri H atom)
OR
For qn with relative rate:
Ans: Combining both the probability factor & the reactivity ratio factor ( given: 1 n 21) ,
the relative ration of A to B = (1)(21) : (9)(1) = 7:3
When propane is chlorinated, the ratio of the 2 prod formed in a 1:1 ratio instead. Suggest an explanation for the diff b/w this ratio and the expected ratio from (a)
The expected ratio is obtained by assuming that all H atoms have the same reactivity. (A) is formed from .CH2CH2CH3 intermediate while (B) is formed from CH3.CHCH3, which is a 2° radical that is more stable than .CH2CH2CH3, a 1° radical. Due to the presence of 1 more e- - donating alkyl grp attached to the e- deficient C atom, there’s a greater proportion of (B) than expected.
