74
votes
Accepted
Why does F replace the axial bond in PCl5?
Recently, there has been a lot of discussion of Bent's rule (see for example "What is Bent's rule?") here in SE Chem. Simply stated, the rule suggests that $\mathrm{p}$-character tends to ...
- 82.3k
57
votes
Accepted
Why is fluorine a gas, but lithium isn't?
A lithium atom has one valence electron, easily lost (shared), so it is connected to other atoms by a metallic bond. This is a bit like the shell game where a pea (electron) is hidden under a walnut ...
- 24.4k
47
votes
Why do compounds like SF6 and SF4 exist but SH6 and SH4 don't?
TL;DR Fluorine is electronegative and can support the extra negative charge that is dispersed on the six X atoms in $\ce{SX6}$, whereas hydrogen cannot.
First, let's debunk a commonly taught myth, ...
- 66.4k
47
votes
Accepted
What is the molecular structure of xenon hexafluoride?
TL;DR Xenon hexafluoride has a fluxional structure in the gas phase, with multiple rapidly interconverting conformers. The three most important conformers have $C_\mathrm{3v}$, $O_\mathrm{h}$, and $C_\...
- 66.4k
37
votes
Storing hydrofluoric acid before the invention of plastics
In the original 1771 experiment, Scheele used a very simple setup consisting of a glass retort with a glass receiver (round-bottom flask).
Yes, the glass was etched to some degree by the fumes, but ...
- 35.7k
36
votes
Accepted
Why does alcoholic KOH prefer elimination whereas aqueous KOH prefers substitution?
In alcoholic solution, the $\ce{KOH}$ is basic enough ($\mathrm{p}K_{\mathrm{a}} =15.74$) to deprotonate a small amount of the alcohol molecules ($\mathrm{p}K_{\mathrm{a}}= 16–17$), thus forming ...
- 17.3k
25
votes
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Why is sulfur hexafluoride more stable than selenium or tellurium hexafluoride?
$\ce{SF6}$ is extremely stable for purely steric reasons, because S is completely blocked by fluorine atoms from all directions, so the reactions starting with an attack on S that otherwise would ...
- 30k
22
votes
Accepted
Why is WF6 stable whereas CrF6 is unknown?
The answer has to do with two things. Note that HSAB theory is dubious at best and doesn't have very useful predictive power, so I am going to avoid talking about it.
(1) The accessibility of the high ...
- 66.4k
21
votes
Accepted
Reasons for negative iodoform test
As @Waylander pointed out, it appears this reaction has not been performed and/or recorded in any literature, so it is quite dangerous to speculate.
But keeping that aside, A 3D perspective reveals ...
- 2,665
21
votes
Accepted
Can fluorine ever have a positive oxidation state?
As noted in the referenced question, fluorine is not in the +1 oxidation state in hypofluorous acid (which, incidentally, is the only hypohalous acid that has been isolated) nor in any other compound ...
- 41.2k
20
votes
Can fluorine act as the central atom in interhalogen compounds?
Why can't fluorine be the central atom in inter-halogen compounds?
First off, fluorine can be at the "center of things." Examples would include the strongly hydrogen bonded hydrofluoric acid
and ...
- 82.3k
20
votes
Accepted
Enthalpies of formation of alkali metal halides
The equation for the formation of $\ce{MX(s)}$ from its constituent elements is:
$$\ce{M(s) + 1/2 X2(g) -> MX (s)}$$
In general, $\ce{X2}$ could be liquid ($\ce{X} = \ce{Br}$) or solid ($\ce{X} = ...
- 66.4k
20
votes
Accepted
Is there a haloalkane for astatine?
Part of the reason why there has been very little research done on organoastatine chemistry is the extreme radioactivity of astatine; the half-life of $\ce{^210At}$, the longest-lived isotope, is ...
- 1,100
19
votes
Accepted
Is HF the least acidic hydrogen halide?
First of all as @chipbuster says $\ce{HF}$ in diluted solutions in water is nearly completely dissociated and therefore shouldn't be called weak. Wikipedia describes this nicely and cites several ...
- 4,414
19
votes
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How to rationalise the planar structre of I2Cl6?
The bonding situation in the compound $\ce{(ICl3)2}$ is by far more complex than what is depicted in this book. The molecule itself has very high symmetry, i.e. $D_\mathrm{2h}$, that needs to be ...
- 42.4k
19
votes
Accepted
What is hybridisation of XeF6 in solid state?
Forget about applying hybridization outside the second row, especially in 'hypervalent' compounds. I know, that it is common to use and sometimes works, but it is incorrect.
The $\ce{XeF6}$ molecule ...
- 20.7k
19
votes
Accepted
Why does chlorination of toluene stop at two chlorine atoms?
It doesn't stop, it just gets a lot slower the more chlorine atoms you add. From the abstract of an article titled Photochemical side-chain chlorination of toluene[1]:
Where $k_1$, $k_2$, and $k_3$ ...
- 23.4k
18
votes
Accepted
Why do XeO and XeF8 not exist?
$\ce{XeF8}$ is not known to exist though O.N is +8. Why is this so?
At least 2 compounds have been reported that contain the $\ce{XeF8^{2-}}$ unit. See, for example:
$\ce{(NO^+)2[XeF8]^{2-}}$ (...
- 82.3k
17
votes
Why is thionyl chloride preferred for preparing alkyl chlorides from alcohols?
Preparation of alkyl chlorides from alcohols by thionyl chloride is preferred over other methods mentioned because of the following reasons :
$$\ce{R-OH + PCl5 → R-Cl + POCl3 + HCl ↑}$$
$$\ce{...
- 1,379
17
votes
Accepted
Why is the dipole moment of chloromethane larger than the dipole moment of fluoromethane?
Dipole moment is not just about charges, it also has $L$ term. Bond length of $\ce{C-Cl}$ is greater than $\ce{C-F}$ and in this case, that is more dominating factor.
The dipole moment is in order
...
- 1,000
16
votes
Use of aqueous KOH and alcoholic KOH in dehydrohalogenation reactions
Aqueous $\ce{KOH}$ is alkaline in nature i.e. it dissociates to produce a hydroxide ion. These hydroxide ions act as a strong nucleophile and replace the halogen atom in an alkyl halide.
$$\ce{RCl + ...
- 929
16
votes
Accepted
Why is thionyl chloride preferred for preparing alkyl chlorides from alcohols?
Thionyl chloride is preferred for preparing alkyl chlorides from alcohols because the by-products formed in the reaction are $\ce{SO2}$ and $\ce{HCl}$ which are in gaseous form and escape into the ...
- 1,316
15
votes
Accepted
Will this beta-branched secondary alkyl halide undergo SN1 or SN2?
Secondary alkyl halides are on the borderline of $\ce{S_{N}2}$ and $\ce{S_{N}1}$, so either could be operating. Branching at the position beta to the halide will further hinder the backside attack ...
- 10.5k
15
votes
Accepted
Storing hydrofluoric acid before the invention of plastics
Based on research inspired by andselisk's answer, chemists stored it in glass vessels coated in wax (similar to the receiver setup Scheele used to prove the silicon dioxide precipitate was from the ...
- 266
14
votes
Accepted
How can the hydrogen bonds in solid HF be best represented?
At Wikipedia you find this structure (drawn by Benjah-bmm27 on Wikimedia Commons):
In the crystal, there is a hexagonal lattice of these chains, so the packing of the fluorine (green spheres) has an ...
- 17.3k
14
votes
Accepted
Why isn't a polar protic solvent used in the Finkelstein reaction?
The key point is driving the equilibrium where you want it. You could do it by relative reaction rates. The Finkelstein reaction, however, uses a better method: precipitation of one of the products. ...
- 64.4k
14
votes
Accepted
Explanation for the trends in nucleophilicity—Orbital interactions or electrostatic attraction
This is a rather intellectually-stimulating question and one that is also very difficult to answer. You have constructed a very good case for why the nucleophilicity order would not be expected to ...
- 9,454
14
votes
Reasons for negative iodoform test
There have been a few answers for this question, which make sense on steric hindrance preventing the idoform formation. However, one thing I wouldn’t agree with is almost everybody’s claim of not ...
- 35.4k
14
votes
Accepted
How can AuCl3 exist?
Gold(III) chloride does not exist as $\ce{AuCl3}$. According to Wikipedia, the name gold trichloride is a simplification of the name, which is referring to the empirical formula, $\ce{AuCl3}$. The X-...
- 35.4k
13
votes
Accepted
Is an SN1 reaction possible in halobenzene?
Yes, that reaction occurs, but generation of the phenyl carbocation was much more difficult than anyone might have guessed. Here is a drawing of the phenyl carbocation.
First note that the 6 p ...
- 82.3k
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