In my textbook, it says that alkanes are generally the least reactive of the three, alkenes are slightly more reactive, and alkynes are even more reactive. However, alkanes have single bonds, alkenes have double bonds, and alkynes have triple bonds. How can the molecule with triple bonds be the most reactive if triple bonds require the most energy to break?
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4$\begingroup$ When you break a triple bond you have to break all three bonds. To convert an alkyne to an alkene you just have to break one bond. $\endgroup$– MaxWMar 19, 2016 at 23:06
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3$\begingroup$ You don't always break both pi bonds and a sigma bond in alkyne reactions. The "weaker" pi bonds can react, but the sigma bond often stays intact. $\endgroup$– SendersReagentMar 20, 2016 at 1:48
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2$\begingroup$ Tables of bond strengths give you energies for all three bonds together, so they look stronger. The other comments have correctly indicated, the bonds react individually. So the first bond of an alkene to react is more reactive than an alkane, etc. $\endgroup$– LighthartMar 20, 2016 at 4:01
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3 Answers
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Maybe it will help if you look at reactivity in terms of ability to attract electrons instead of bond enthalpies. Alkanes are $sp^3$ hybridized, and hence have $25$% $s$ orbital character and $75$% p character. $s$ orbitals are closer to the nucleus and thus have a 'contracting' effect on the hybrid orbital. Greater the $s$ character, 'smaller' the hybrid orbital. This means that electrons are more closely packed. Alkanes have only 25% $s$ character, hence the hybrid orbitals are comparatively larger, and the effective nuclear charge on outermost electrons is less. Alkenes ($sp^2$) and alkynes ($sp$) have 33.3% and 50% $s$ character respectively. Thus their hybrid orbitals are SMALLER; i.e. effective nuclear charge is more. This means that it is easier to accept electrons, as now the effective strength of nucleus is more. Hence, alkynes can easily accept electrons, followed by alkenes and then alkanes. Thus, alkynes are most reactive, followed by alkenes and alkanes. Even though increase in $s$ character increases bond enthalpy, but it also increases the electron withdrawing capacity; and it is the latter which predominates.
(In fact, in general, greater the $s$ character, more the electronegativity and more the acidic nature).
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$\begingroup$ But this is just acidity, and thus only applies to terminal alkynes. There is much more to alkyne reactivity than just acidity. $\endgroup$– JanOct 3, 2016 at 18:56
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The three bonds consist of two pi and one sigma bond.. Pi are very easy to break and sigma is difficult.. So alkynes can be easily broken as they have a more number of pi bonds while alkanes have only one sigma bind which is difficult to break.
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Your text book is wrong alkenes are most reactive and alkynes are most acidic as they have the most s character due to sp-bonds .. The fact that alkenes are most reactive is due to a single pie bond but alkynes have two π-bonds which contribute in the electron delocalising, which would reduce the energy of the π-system
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$\begingroup$ The two $\pi$ systems in alkynes are orthogonal so their energy is not reduced by interaction with each other. Additionally, it is meaningless to say that alkenes are more reactive than alkynes without specifying reactivity towards what. There are reactions that alkenes won't do which alkynes will and vice versa. $\endgroup$– bonJan 11, 2017 at 14:02