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How to compare the general trend in amount of combustion in lower alcohols? (upto five/six carbons) Does the branching or straight chain have any effect on combustion?

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    $\begingroup$ Well all alcohols are not less flammable that all alkanes. The flash point of pure ethanol is 16.60 °C but the flash point of n-icosane is greater than 113 °C. Yes branching effects pretty much everything about a molecule. $\endgroup$ – MaxW Jun 30 '18 at 18:09
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Alcohols as well as alkanes can burn in air if the concentration is high enough. The flash point is the lowest temperature at which a liquid emits vapour in sufficient concentration to form an ignitable mixture with air near the surface of the liquid. Since this process depends on the evaporation of vapours from the liquid, the trend of flash point temperatures roughly follows the trend of boiling point temperatures.

$$ \textbf{} \\ \begin{array}{lll} \hline & \text{Flash point in °C [1]} & \text{Boiling point in °C [2]} \\ \hline \text{Methane}& \text{n.a.} & -161.48 \\ \text{Methanol} & \hphantom{-0}11 & \hphantom{-0}64.6 \\[3pt] \text{Ethane} & \text{n.a.} & \hphantom{0}{-}88.6 \\ \text{Ethanol} & \hphantom{-0}13 & \hphantom{-0}78.29 \\[3pt] \text{Propane} & -104 & \hphantom{0}{-}42.1 \\ \text{Propan-1-ol} & \hphantom{-0}23 & \hphantom{-0}97.2 \\ \text{Propan-2-ol} & \hphantom{-0}12 & \hphantom{-0}82.3 \\[3pt] \text{Butane} & \hphantom{0}{-}60 & \hphantom{00}{-}0.5 \\ \text{Butan-1-ol} & \hphantom{-0}37 & \hphantom{-}117.73 \\ \text{Butan-2-ol} & \hphantom{-0}24 & \hphantom{-0}99.51 \\[3pt] \text{Pentane} & \hphantom{0}{-}40 & \hphantom{-0}36.06 \\ \text{Pentan-1-ol} & \hphantom{-0}33 & \hphantom{-}137.98 \\ \text{Pentan-2-ol} & \hphantom{-0}34 & \hphantom{-}119.3 \\ \text{Pentan-3-ol} & \hphantom{-0}41 & \hphantom{-}116.25 \\[3pt] \hline \end{array}$$

Since alcohols have higher boiling points than the corresponding alkanes, also their flash points may be expected to be higher. The length and branching of the carbon chain can affect the flash point as well as the boiling point.

[1] “Flammability of Chemical Substances”, in CRC Handbook of Chemistry and Physics, 90th Edition (CD-ROM Version 2010), David R. Lide, ed., CRC Press/Taylor and Francis, Boca Raton, FL.
[2] “Physical Constants of Organic Compounds”, in CRC Handbook of Chemistry and Physics, 90th Edition (CD-ROM Version 2010), David R. Lide, ed., CRC Press/Taylor and Francis, Boca Raton, FL.

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