2024-01-30T13:13:25+03:00[Europe/Moscow] en true <p>how do alkanes display isomerism</p>, <p>why bp, mp of alkanes increase as no. of C increase?</p>, <p>why alkanes insoluble in polar but soluble in non-polar solvents?</p>, <p>why density of alkanes increase as no. of C increases?</p>, <p>prepare C<sub>n</sub>H<sub>2n+2</sub></p>, <p>why alkanes generally unreactive, including towards polar reagents?</p>, <p>describe free radical substitution(FRS)</p>, <p>why FRS not a good way to synthesise halogenoalkanes?</p>, <p>how reactivity of FRS differ w/ diff halogens?</p>, <p>what are uses of alkane?</p> flashcards
H2 Chemistry 10 - Alkanes

H2 Chemistry 10 - Alkanes

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  • how do alkanes display isomerism

    cis-trans; restricted rotation about rigid bond; only cyclic alkane as ring structure prevents free rotation of C-C bond

    enantiomer; 1 chiral C, no plane of symmetry

    total no. of stereoisomers = 2n, n=no. of chiral C

  • why bp, mp of alkanes increase as no. of C increase?

    ↑ amount of energy required >

    overcome ↑ strength of id-id attraction b/w molecules >

    as no. of e- per molecules ↑

    straight vs branched

    branching of carbon chain, > spherical >

    < SA of contact b/w adjacent molecules >

    < amount of energy required >

    overcome ↓ strength of id-id attraction b/w molecules

  • why alkanes insoluble in polar but soluble in non-polar solvents?

    energy evolved from formation of id-id attractions b/w alkane, H2O molecules insufficient >

    compensate energy required to overcome stronger H bonds b/w H2O molecules

  • why density of alkanes increase as no. of C increases?

    ↑C increase, mass, vol both ↑ >

    ↑ mass > ↑ vol as >

    ↑ strength of id-id attraction b/w molecules pulls molecules closer >

    become > compact, > dense

    > branching, < density

  • prepare CnH2n+2

    CnH2n + H2 -> CnH2n+2

    rxn:reduction

    r&c:H2(g), Ni catalyst, heat

  • why alkanes generally unreactive, including towards polar reagents?

    electronegativity

    C, H = electronegativity >

    C-H bond non-polar >

    non-polar C-H unreactive to polar reagents >

    strength

    large amount of energy required >

    break C-C, C-H bonds

    thus strong heating, photochemical initiator required for reaction to occur

  • describe free radical substitution(FRS)

    IPT

    r&c: limited X2(g), UV light

    Initiation

    X-X homolytic fission -> 2Xo free radicals

    catalysed by UV light

    Propagation

    limited X2(g)

    Xo FR react w/ alkane, take H atom -> HX + FR alkane missing H atom >

    FR alkane react w/ X2, take H atom -> alkyl halide + Xo FR

    excess X2(g)

    alkyl halide keeps reacting w/ Xo -> HX + FR alkyl halide >

    FR alkyl halide react w/ HX -> di-alkyl halide + Xo FR...

    Termination

    rxn ends when FRs react tgt to form stable pdt

  • why FRS not a good way to synthesise halogenoalkanes?

    substitution of H atom random >

    mixture of mono-subbed, poly-subbed pdts formed >

    < yield of required pdt

  • how reactivity of FRS differ w/ diff halogens?

    group down halogen group, reactivity decreases >

    BE(bonds broken) - BE(bonds formed) = enthalpy change of rxn >

    X-X bonds stronger, H-X bonds weaker >

    enthalpy change of rxn > endothermic, < energetically preferred

  • what are uses of alkane?

    commercial fuel;

    as natural gas, (a) formed by decomposition of organic matter by methanogens/ (b) fossils as organic matter decomposes underground

    +ve: (a) renewable vs fossil fuels

    -ve: contributes to global warming

    as liquified petroleum gas(LPG)

    propellants/refrigerants;

    CnH2n+2 replace CFCs

    +ve: doesnt deplete ozone, reduce global warming

    -ve: flammable