Wikipedia:Reference desk/Archives/Science/2021 January 19

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January 19

substitution reactions of alcohols with halides to form alkyl halides

Someone told me that in the substitution reactions of alcohols with halides to form alkyl halides, tertiary alcohols react faster than secondary alcohols and primary alcohols. Why is that so?Jocosus2000 (talk) 08:12, 19 January 2021 (UTC)

That transformation goes by a mechanism called SN1 reaction in which the intermediate is a carbocation. The stability of these cations goes in the sequence primary<secondary<tertiary, hence the fastest example is where that cation is tertiary. Note that in practice the reaction needs very strong acid and possibly a dehydrating agent to remove the water that is formed from the alcohol. Otherwise it can go in the reverse direction (i.e. alcohols can be made from alkyl halides, as the SN1 reaction article describes). Mike Turnbull (talk) 11:36, 19 January 2021 (UTC)

Thanks, will there be SN2 reaction? If there is SN2 reaction then primary alcohols will be faster...Jocosus2000 (talk) 11:55, 19 January 2021 (UTC)

SN2 reactions are almost always used when starting with alkyl halides and going to something else. Alcohols themselves don't react in SN2 reactions unless first transformed into a good leaving group (e.g. tosylate). But you are correct that for SN2 reactions the rate of reaction is usually primary>secondary, with tertiary essentially unreactive by that mechanism. Any decent organic chemistry textbook will expand on this if the Wikipedia articles aren't enough. Mike Turnbull (talk) 13:55, 19 January 2021 (UTC)

Okay. thank you so much!Jocosus2000 (talk) 00:19, 20 January 2021 (UTC)

So, the reaction goes by SN1, but not SN2 ( even in primary alcohols)?Jocosus2000 (talk) 04:57, 20 January 2021 (UTC)

Yes, the C-O bond in alcohols is too strong to permit the Sn2 reaction to take place. 2601:646:8A01:B180:6D2B:1920:798B:506B (talk) 06:33, 20 January 2021 (UTC)

What makes General relativity is different from Quantum mechanics?

What makes General relativity is different from Quantum mechanics? Rizosome (talk) 14:25, 19 January 2021 (UTC)

Wow. That's a BIG QUESTION. The best way I can answer it is thusly: Modern quantum mechanics is fundamentally about quantized fields, see Quantum field theory for the gory details, but in VERY SIMPLIFIED version, a quantum field is a field that is quantized. I know that's not much help, so here's what each of THOSE means: a field is a value which is assigned to every point in space. That value can be a simple one-part number (called a scalar), a two-part number called a vector, or a multi-part number called a tensor; but regardless of which it is, you just need to think about the field as simply a value assigned to every point in space. For example, an electric field is a set of values assigned to every point in space telling you how a point charge will move if it were located at that spot. A field is quantized if the values assigned to that spot have a functional minimum size called the quantum of that property. Values of the field cannot be smaller than that quantum value, and all values need to be a multiple of that quantum value (i.e. no "in-between values"). General relativity states that gravity is not caused by a force, but rather by curved spacetime. The fundamental problem is that each theory contains an axiom which makes it incompatible with the other theory, and that attempts to take that axiom out breaks the theory in question. For example, quantum mechanics assumes a flat, continuous spacetime; space and time are not quantized per se, it's the values that are assigned to the fields in space that are quantized; space itself in QM is continuous and flat. In order to have quantum gravity, you'd have to have quantized spacetime itself; i.e. some fundamental unit of spacetime which cannot be further subdivided. That introduces all sorts of paradoxes into both theories. I've probably oversimplified and glossed over some stuff here, but Here is a video from PBS Space Time which is at what I would call a "High Layperson" level (some background vocabulary knowledge and exposure to the basics of these theories needed, but no math involved). If you want some good "low level" coverage of modern physics (for the real layperson, with no background) the BEST stuff can be found at The Science Asylum, whose explanations are fantastic even for people with no background in the subject. He has a NUMBER of great videos on both General Relativity and Quantum Theory. His videos on Quantum Fields, for example, would be a great overview for what I tried to explain above. --Jayron32 17:01, 19 January 2021 (UTC)

A very big difference is that GR is a classical theory, where you can predict the evolution of a system by solving some differential equations, like in Newtonian mechanics except the equations are more complicated. QM is different because measurement outcomes are probabilistic. No one understands what is really going on "underneath", and explaining it is called the (unsolved) measurement problem. 2601:648:8202:96B0:0:0:0:313A (talk) 22:01, 19 January 2021 (UTC)

I guess the question is really meant to be about the fundamental incompatibility between the two theories, referred to in many of our articles, and dealt with (somewhat perfunctorily) halfway the first response.  --Lambiam 08:59, 20 January 2021 (UTC)

Too much is made out of that. It's mostly based on convenient mathematical formalisms used in QFT where you have a flat space-time and have a specialized formalism involving in and out states not working, rather than a fundamental problem. See here for details. Count Iblis (talk) 11:03, 20 January 2021 (UTC)

Who won Thorne–Hawking–Preskill bet at the end?

Who won Thorne–Hawking–Preskill bet at the end? Wiki article didn't mention any winners clearly. Rizosome (talk) 16:38, 19 January 2021 (UTC)

It hasn't been resolved yet. The black hole information paradox has not been fully resolved in a way that would satisfy the terms of the bet. Some recent developments were published in 2019 that the Wikipedia article oversells a bit; the significance of these new developments have not been fully explored, and even if they resolve some of the issues, they do not resolve all of them. --Jayron32 17:05, 19 January 2021 (UTC)