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Wikipedia:Reference desk/Archives/Science/2016 December 3

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December 3 edit

Is Glicine not chiral? edit

In this video (10:20>) the woman says that the Glycine is not chiral because it has 2 different group only rather than four. Then I can understand from her things that amino acid can be chiral just in case that it has 4 different groups. But I don't understand why when even it has only two different groups it can not be chiral, here is the group of the left side can be on the right and vice versa and it is chiral since it has a mirror image as well. 93.126.88.30 (talk) 02:00, 3 December 2016 (UTC)[reply]

This is one of those questions where it would help if you had something physical to model the molecule with, like any stick-and-ball molecule kit. If you were to build both glycine and any mirror-image, you would find that you could make the two molecules identical simply by rotating bonds or the whole model around, and therefore conclude that all mirrored forms of glycine are actually glycine. Someguy1221 (talk) 02:13, 3 December 2016 (UTC)[reply]
I do have a physical model here and I still don't understand why it's not chiral if you rotate it the same as doing with the hand. 93.126.88.30 (talk) 02:58, 3 December 2016 (UTC)[reply]
I'm sorry, I guess I don't fully understand what the hangup is. If you take like, clay or plastic models of the left- and right-hand, it's quickly obvious that no amount of rotations will make them identical. Someguy1221 (talk) 03:20, 3 December 2016 (UTC)[reply]
Take a pair of gloves that have different "left" vs "right" gloves. Put the left one on your left hand. Now take it off and the left one on your right hand (or instead now put the right glove on your left hand). If these results feel different to you, then your hands are not identical. DMacks (talk) 10:40, 3 December 2016 (UTC)[reply]
Note that "two different groups" is not correct for this structure, nor is it what is needed in this context. The requirement for chirality of this sort of structure is "all four different", so "any two (or more) the same" makes it nonchiral because that would mean there would only be less than four different. DMacks (talk) 10:44, 3 December 2016 (UTC)[reply]
There is no carbon atom with 4 different things attached. If you put a mirror in the plane of the carbon atoms and oxygens, you can see that its reflection is identical. Graeme Bartlett (talk) 11:19, 3 December 2016 (UTC)[reply]
The physical model might mislead you. The thing to bear in mind with chirality is that it's not merely whether you can pick up a model molecule and tell if you've inverted it in a mirror - can you tell if you've inverted it after you allow every bond that rotates to do so freely? The mirror image glycine may look different, but I guarantee you can twist some bonds around and get back where you started. You might play with [1] which is a free open source chemical modeling program.
(Annoying pedantry follows) Note that it is possible, rarely, to have a chiral molecule without any chiral centers because its geometry does not allow free rotation - for example, C(CH3)=C=C(CH2CH3)(a cumulated diene) has no carbons with four distinct ligands, but it is non-superimposable if the ethyl goes right instead of left - that is an enantiomer since you can see it would be a mirror image to swap those. I imagine that there are some compounds that are achiral at room temperature because we assume that their conformation changes constantly, yet become chiral if they are kept at cryogenic temperatures because they no longer can access the higher-energy states needed to rotate all sigma bonds freely. Wnt (talk) 12:06, 3 December 2016 (UTC)[reply]
Atropisomers have the thermal/kinetic barrier to rotation on a single bond you describe. These and the allene case are examples of axial chirality. DMacks (talk) 20:58, 3 December 2016 (UTC)[reply]
It's not even necessary to have π systems around for a molecule to be chiral. Spiro compounds can also be chiral. (Imagine a molecule with two pyrrolidine rings connected at the carbon to the left of each nitrogen in the ring, and with the other carbon next to this connecting point double-bonded to an oxygen atom. The central carbon must be tetrahedral, so the rings are orthogonal and there can be no plane of symmetry.) A molecule with a plane of symmetry or centre of symmetry cannot be chiral, but a molecule with an axis of symmetry can be. Double sharp (talk) 04:54, 4 December 2016 (UTC)[reply]
This description is confusing...do you mean two 2-pyrrolidones that form a spiro compound by virtue of sharing single carbon as their respective C3 positions? That sort of chirality thing is mentioned at the spiro compound article. DMacks (talk) 21:06, 4 December 2016 (UTC)[reply]
Yes, that's the one! Thank you and sorry for the obtuse description! Double sharp (talk) 03:22, 5 December 2016 (UTC)[reply]
2,​7-​Diazaspiro[4.4]​nonane-​1,​6-​dione (CAS# 36817-39-7) if anyone is interested. DMacks (talk) 06:06, 5 December 2016 (UTC)[reply]
 
1,​7-​Dioxaspiro[5.5]​undecane
One of my teaching examples is 1,​7-​dioxaspiro[5.5]​undecane, a sex pheremone of the olive fruit fly, where one enantiomer is active against males and the other against females. For a review, see Mori, Kenji (2011). "Chirality and bioactivity". Chimica Oggi. 29 (4): 40–43. DMacks (talk) 21:32, 4 December 2016 (UTC)[reply]
I should add we have an article on glycine, though it lacks the usual pretty picture. See [2] for that. Wnt (talk) 12:43, 3 December 2016 (UTC)[reply]

What does the word "cis" (of cis isomer) stand for? edit

I know what it is cis but I don't know what is the meaning of the word itself.93.126.88.30 (talk) 03:00, 3 December 2016 (UTC)[reply]

It means "on this side of", the OED has "Latin cis prep. ‘on this side of’, opposed to trans or ultra, across, beyond; also used in comb. as in cis-alpīnus, cis-montānus, lying on this side the Alps or the mountains, cis-rhenānus on this side the Rhine, cis-tiberis on this side the Tiber. The two first of these esp. continued in use in medieval Latin in reference to Rome and Italy, whence Italian cisalpino, French cisalpin, cismontain, cisalpine adj., cismontane adj." and for the chemical sense "Designating a compound in which two atoms or groups are situated on the same side of some plane of symmetry passing through the compound; cis-trans isomerism, a form of isomerism in which in one isomer two identical groups are on the same side of the plane of a double bond whereas in the other isomer they are on opposite sides; so cis-trans isomer". See also Cis–trans isomerism for discussion of its use in chemistry. DuncanHill (talk) 03:16, 3 December 2016 (UTC)[reply]
Thank you. I've read the article here before I asked my question. Now I understand that the meaning of "cis" in Latin is "on the same side". I think that such essential information should be in the article which talks about the cis isomer.93.126.88.30 (talk) 04:44, 3 December 2016 (UTC)[reply]
If you mean the article Duncan cited, it's there in the first paragraph:
The terms “cis” and “trans” are from Latin.
--76.71.5.45 (talk) 05:24, 3 December 2016 (UTC)[reply]
And does that sentence tell you what the word means? —Tamfang (talk) 20:32, 3 December 2016 (UTC)[reply]
Nowadays "cisgender" is probably the cis- heard most often outside of chemistry. Wnt (talk) 11:46, 3 December 2016 (UTC)[reply]
The Cissy is a tangentally related South Park episode. StuRat (talk) 13:44, 4 December 2016 (UTC) [reply]

What do the letters Pk stands for in chemistry (acids & basis)? edit

I know the meaning but not what each letter stands for. By gogleing I didn't find the answer.

93.126.88.30 (talk) 04:43, 3 December 2016 (UTC)[reply]

p = -log, K = the constant for which you want to find the -log. 2601:646:8E01:7E0B:4194:43D8:DAB4:1C6C (talk) 05:53, 3 December 2016 (UTC)[reply]
It might stand for concentration of potassium K. See pH. Dolphin (t) 06:17, 3 December 2016 (UTC)[reply]
PH#History discusses the historical meaning of "p". Sørensen never stated exactly why he chose "p", but it is generally thought to mean logarithmic power, as the IP above me stated. "K" has a long history of being used as a reaction constant in chemistry, at least since 1864 (see Law_of_mass_action#History), though I haven't found any explanation as to why the authors chose K, though it could be as simple as the fact that "constant" in Norwegian (the native language of the first author I know to have used it) is konstant. Someguy1221 (talk) 06:22, 3 December 2016 (UTC)[reply]
In my chemistry class I remember it stood for "puissance", the French word for power. Doesn't mean that's right though. Graeme Bartlett (talk) 11:15, 3 December 2016 (UTC)[reply]
And similarly I remember hearing that it was from the German Potenz. That one is somewhat appealing because it means "power" in the mathematical sense, as opposed to the other senses of the English word (which would be instead probably Kraft or Macht). OTOH, there have been so many explanations of the "p" notation that I think it's safe to say that "nobody knows, but 'power of hydrogen' is as good an answer as any". Double sharp (talk) 05:09, 4 December 2016 (UTC)[reply]
I assume everyone is talking about pKa, not p[K+] or pH or p[H+] etc., though that is indeed the place to look about "p" notation. It is the inverse log (p) of the acid constant, a type of equilibrium constant. Wnt (talk) 11:44, 3 December 2016 (UTC)[reply]

What the heck is a "wedge connector", and should there be a Wikipedia article about it? edit

Hello RD/S folks,
I deleted the article "Wedge connector" as a copyvio/advertising only page. Copyvio/advertising only pages I know about. Electrical connectors, not so much. And by "not so much", I mean that "me and a soldering iron" might be a plausible redirect to WP:CIR.
I've googled, and would appear to me that this thingumabob is a type of connector for power lines, particularly near electrical substations.
Should there be an article, or section in an article, about "wedge connectors"?
Pete AU aka --Shirt58 (talk) 09:12, 3 December 2016 (UTC)[reply]

Hi Shirt58 it's a "widget" used on high voltage powerlines. I found a few possible sources that might be able to start an article:
Roger (Dodger67) (talk) 09:41, 4 December 2016 (UTC)[reply]

24v from PSU edit

On this page someone commented;

To get 24vdc, you can use +12 as positive terminal, and use -12 as ground terminal..voltage is potential difference, thats might be the reason you dont get voltage from second psu unit-no difference(+12v vs +12vs difference is 0v)..imagine two AA batteries connected in series..it will produce 3v, but actually you can tap the middle point as GND and came up with -1.5v on negative & +1.5v on positive terminal..that is basically concept of terminal +VE,-VE, & GND..

That explanation went entirely over my head. Please explain in idiot terms how to get 24v from a PSU that has 12v and -12v lines? I know nothing about electricity theory. — Preceding unsigned comment added by 125.141.200.13 (talk) 18:30, 3 December 2016‎ (UTC)[reply]

The difference between 12V and –12V is 24V (basic math, nothing to do specifically with electricity). So if you call the –12V line the "ground" for your circuit, then the 12V line is 24V higher than it. DMacks (talk) 20:54, 3 December 2016 (UTC)[reply]
Eh? 12 minus -12 is 0 isn't it? 125.141.200.13 (talk) 00:55, 4 December 2016 (UTC)[reply]
No. Maybe not having a good understanding of the idea of Negative numbers and Arithmetic involving negative numbers is the key to your original confusion? DMacks (talk) 03:50, 4 December 2016 (UTC)[reply]
The OP is wrong in subtracting because the two supplies are effectively in series, as in his illustration with the two AA batteries, so their voltages should be added, not subtracted. So it's 12 plus 12. Akld guy (talk) 18:13, 4 December 2016 (UTC)[reply]
Sorry for being dense but wouldn't -12 + 12 also be 0? 125.141.200.13 (talk) 00:59, 5 December 2016 (UTC)[reply]
No. The +ve of the negative supply is connected to the -ve of the positive supply, just like the two series-connected AA cells, so the voltages add. The total voltage from the -ve terminal to the +ve terminal is 24V. Akld guy (talk) 06:03, 5 December 2016 (UTC)[reply]
This usually works just fine, but some power supplies get confused and shut down. This is especially likely when the +12V puts out more current than the -12V -- the -12V circuit might not like being driven backwards like that. --Guy Macon (talk) 21:06, 3 December 2016 (UTC)[reply]
Yes, the negative lead of the 24V device can be connected to the -12V line and the positive lead to the +12V line. All will be well, with a couple of caveats: there must be no DC path of any kind from the 24V device to the power supply's common ground (the mid-point). This applies to all inputs and outputs of the device. So for instance, if a 24V amplifier is connected in this way, its input connection must have no common path (called a ground loop) via its own wiring or via any other device, to that mid-point. Such a path would probably exist with a 12V preamplifier ahead of a 24V amplifier; the common ground of the preamp and amplifier would result in a path through the preamp, probably causing serious damage. Also, if the case of the 24V device is internally connected to one of the device's DC leads, great care must be taken that it does not accidentally come in contact with the mid-point or with the case of some associated device connected to the same power supply. For audio devices, an audio transformer will provide the required isolation, but of course the characteristics of the transformer must be chosen so as to not impair the frequency range and other performance aspects of the device. Akld guy (talk) 21:25, 3 December 2016 (UTC)[reply]
  • It's not a well-written page and they don't seem to appreciate the several drawbacks to this. A 10Ω 10W load resistor on the 5V rail is a bit crude and it will be a full 10W load, so that's a bunch of heat to get rid of.
One issue in particular is that -ve supplies on PC power supplies are just there to support low current devices, originally serial ports. These supplies are completely unbalanced, in terms of how much current the +ve can supply vs. the -ve. So that if you use a PC supply as a 24V supply, it "works" (with the ground offset problem mentioned above), but it will be a great big PSU that still only achieves a very low useful current.
If you need a 24V supply, then my route these days is eBay and the vast range of cheap DC-DC boost converters now available. Often based on the LM2596 and LM2621 ICs. They're cheap, they deliver several A and they only cost a few quid. Andy Dingley (talk) 21:45, 3 December 2016 (UTC)[reply]
I should point out that 10R at 5V is 2.5 W, not 10. V2/R. Tevildo (talk) 22:08, 3 December 2016 (UTC)[reply]

Science or Science Fiction? Professionals’ Discursive Construction of Climate Change edit

I saw this in my Twitter feed: http://oss.sagepub.com/content/33/11/1477.full Is this a reputable journal? Is it a reputable article? It's being cited to back the claim that the majority of scientists are skeptical of global warming "crisis". A Quest For Knowledge (talk) 19:35, 3 December 2016 (UTC)[reply]

The claim is bogus for it was not a representative survey of scientists even according to the authors of the study. --Modocc (talk) 20:55, 3 December 2016 (UTC)[reply]
It certainly is bogus for supporting the claim that the majority of scientists are skeptical of a global warming crisis, but would be a reliable source concerning what was actually studied, which the paper describes as:
"To address this, we reconstruct the frames of one group of experts who have not received much attention in previous research and yet play a central role in understanding industry responses – professional experts in petroleum and related industries." and "To answer this question, we consider how climate change is constructed by professional engineers and geoscientists in the province of Alberta, Canada"
It is the improper changing of "professional engineers and geoscientists in the province of Alberta, Canada" to "all scientists" that is completely bogus. --Guy Macon (talk) 21:19, 3 December 2016 (UTC)[reply]
Yep on a quick read of the article I agree. It is just a study of how people don't want to think they are part of a problem and like to stand up for the company and industry they work for. Not exactly surprising, it is the strength and extent of the effect most people don't understand. Even scientists are unfortunately prone to that - it is one of the things they have to learn to guard against and work hard to maintain. Dmcq (talk) 12:28, 4 December 2016 (UTC)[reply]
"It is difficult to get a man to understand something, when his salary depends upon his not understanding it!" - Upton Sinclair, American writer. Wikiquote. Carbon Caryatid (talk) 17:22, 5 December 2016 (UTC)[reply]

Fishing spider in the house edit

I found a fishing spider in my kitchen. We imported some of these in the house as stowaways on the Christmas tree a few years ago, but I'm guessing they live in the garden or on the outside walls as I rarely see them, and this one could've got in while the balcony door was open. I suppose it might have come in to protect itself from the cold (it's 0 to -5 C outside and 14 C in the kitchen). I can't find much info about this species, aside from guessing that it won't be eating much here, considering it feeds on passing insects. Do you think it'll have better chances of survival outside? FWIW I'm pretty sure it's a female as it's the biggest one I've ever seen. 93.142.67.37 (talk) 22:28, 3 December 2016 (UTC)[reply]

I know next to nothing about this spider, but I believe that in general, animals behave in a way that is best for them. Of course they sometimes will not, but that is the basis of natural selection. We could start a debate about the morals and ethics of "interfering" with wildlife, but if you have no objection to her presence in your residence, I would just leave her alone. DrChrissy (talk) 22:45, 3 December 2016 (UTC)[reply]
Alright, thanks. I like spiders anyway, I'm just afraid that she might starve here. I'm going to leave a window cracked open overnight in case she decides she doesn't like it here. 93.142.67.37 (talk) 00:10, 4 December 2016 (UTC)[reply]
I like spiders too. I rarely brush away their cobwebs in my flat and I am always amazed they manage to find food here. Maybe one of our entemological friends can tell us how much/frequently they need to feed on. The open window sounds like a great idea. DrChrissy (talk) 00:30, 4 December 2016 (UTC)[reply]
It may be better to relocate spiders back outside to ensure that they might at least have a chance of producing their own progeny. A modern home, may offer a few prime spidery [des res] locations. Yet, if the food is not there they die within months. One only has to look around a bachelor pad (whose owner is lackadaisical when it comes to using a feather duster) that there are not many spidery des res locations in a house ( in comparison with a garden). So if one see a spider, where one saw one the year before, it is probably a different spider that’s moved in to occupy that exact same spot. A UV florescent dye is available that can be applied to such small critters with a artists paint brush to identified them as individuals, (up and till the time they moult that is). Got no references to hand but I think they only moult once a year unless they are getting lots to eat and I think they can only survive months without any subsidence in a dry, heated environment. The UV dye will also make it easier to locate the shrivelled up remains, that air currents may cast into a dark and dusty corner; post-mortem. Some ecologists have argued ( and me thinks convincingly) that the abundant wild life around the Chernobyl nuclear plant is not evidence of wild life thriving there and producing their own progeny but rather shows that as soon as an ecological niche becomes vacant, due to the failure of the indigenous species successfully breed , wildlife from outside the area quickly move in to fill the emptying niche . So, one may be ignorantly allowing ones home to become a place of solitude and slow starvation by inaction as I don't think an open window will provide a sufficient hint.--Aspro (talk) 18:38, 4 December 2016 (UTC)[reply]
Well, we stopped cleaning up spiderwebs several years back and since then we've had far less problems with colds and allergies, not to mention less flies and mosquitoes, so I think they've got to be eating something after all. Also, the tree incident happened three years ago and I had never seen a fishing spider before that in my life, so I think this is likely one of the spiders that arrived then or their offspring, and not a random neighborhood spider who wandered inside. 93.142.67.37 (talk) 19:20, 4 December 2016 (UTC)[reply]
Personal observations are important and valuable, yet one must be remain aware that this was not a controlled trial and so other factors could be at work here. Glad to hear you're suffering less colds and allergies and in modern draft proofed homes, leaving a window is important in this respect. Especially, if you're cohabiting with young children, which are well known to be very productive virus factories. Cold viruses just love draft free, dry, oxygen deficient environments which causes their hosts mucous membranes to dry out. So continue to keep a window or two open. Not high fan-lights, as they let the hotter air out but the lower ones. An ioniser is also cheap to run and settles both dust and other allergy inducing flotsam an' jetsam that floats around in the air. --Aspro (talk) 20:12, 4 December 2016 (UTC)[reply]
It's probably not going to end up like this case. Count Iblis (talk) 01:16, 4 December 2016 (UTC)[reply]
I've just done a quick search of how often spiders eat. I came across this article[3] which states they (of course there will be species differences) eat 2,000 insects/year. I am really surprised by how high this is as it represents 5.5 insects/day! The other possibility is that she moved in to hibernate - see here[4]. DrChrissy (talk) 20:35, 4 December 2016 (UTC)[reply]
Could be, I haven't seen her since last night. Although, it says here that immature adults hibernate, but this one had a leg span well over 5 cm. 93.142.67.37 (talk) 00:21, 5 December 2016 (UTC)[reply]
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