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There is no difference in boiling point of 2-fluoropentance and 3-fluoropentane, so is chloro- and bromo-. But why is there a difference in the boiling point of 2-iodopentane and 3-iodopentane, up to 10 degree Celsius?

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    $\begingroup$ Well I think that the most intriguing part is that 2-fluoro(chloro, bromo)pentane and 3-fluoro(chloro, bromo)pentane have the same boiling points! That is much more incredible!! $\endgroup$ – SteffX Feb 24 '19 at 15:09
  • $\begingroup$ I can only speculate: iodine is larger and more polarizable, resulting in stronger vdW interactions and more steric (entropic) constraints. This leads to significantly different packing (on average) and vdw interactions. $\endgroup$ – Buck Thorn Feb 24 '19 at 15:44
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    $\begingroup$ @SteffX I'm pretty sure OP's "no difference" means just smaller difference. $\endgroup$ – Mithoron Feb 24 '19 at 20:07
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I have spent a quite a time to figure out "so-called" irrational behavior of the boiling points of iodoalkanes compared to relevent othe halpalkane, but I have seen noting really radical, what OP claimed there are. The following chart listed boiling points of all possible long chain halides of pentane (all have been measured at $\pu{760 mmHg}$): $$ \begin{array}{ccc} \text{locant} &\text{fluoropentane} & \text{chlororopentane} & \text{bromoropentane} & \text{iodopentane}\\\hline 1 & \pu{62.8 ^\circ C} & \pu{108 ^\circ C} & \pu{130 ^\circ C} & \pu{155 ^\circ C}\\ 2 & \pu{58.4 ^\circ C} & \pu{99 ^\circ C} & \pu{117 ^\circ C} & \pu{150 ^\circ C}\\ 3 & \pu{59.5 ^\circ C} & \pu{97.5 ^\circ C} & \pu{119 ^\circ C} & \pu{145 ^\circ C}\\ \end{array} $$

Please be aware that, all of these values are extracted from EPA(USA) website. Also note that values of 2-fluoropentane and 3-fluoropentane are predicted on EPA, but ChemSpider gave experimental values of $\pu{56 ^\circ C}$ and $\pu{56.7 ^\circ C}$, respectively. Also, the boiling point of 3-iodopentane ($\pu{145 ^\circ C}$) has also been predicted, but it is available commercially at Alfa Aesar, which listed the boiling point of it as $\pu{146.6 ^\circ C}$. Thus, these listed values are very reliable. Therefore, I'd say that the values OP has must be not so reliable. I admit that I have to dig deeper to find a reliable source of those predicted values. All three bromo compounds have no problem because all of them are available in Sigma-Aldrich.

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