organic chemistry - If you introduce a 3rd group into O- chlorotoluene, which position will the 3rd group prefer?

I read that Cl is ring deactivating group even if it is O,P- directing. Which group among the methyl and chloro controls the orientation of the 3rd substituent. What are the products formed?

bond - PBP vs TBP geometry?

Why are the axial bond lengths greater than those of the equatorial bonds in a trigonal bi-pyramid (TBP) geometry molecule; but the opposite is true for pentagonal bi-pyramid (PBP) geometry molecules? I think that it might have something to do with the angles between the bonds, so I thought of a possible explanation.

In a TBP molecule, the equatorial bonds are spaced apart from each other by 120 degrees, resulting in lesser repulsion; compared to the axial bonds at 90 degrees. The reduced repulsion leads to greater stability of the equatorial bonds.

In contrast, the equatorial bonds in a PBP molecule are separated by 72 degrees which results in them being less stable when compared to the axial bonds.

To what extent is this reasoning correct? Please provide a more detailed and accurate explanation, if there is one. Thanks!!

Answer

General Rule #1: Most elements use only s and p orbitals to form bonds, only transition elements and heavier elements use d, f, etc. orbitals in bonding.

General Rule #2: The more s-character in a bond the shorter the bond (reference). For example

• a $\ce{C(sp^3)-C(sp^3)}$ single bond length is ~ 1.54 $\mathrm{\mathring{A}}$


• a $\ce{C(sp^2)-C(sp^3)}$ single bond length is ~ 1.50 $\mathrm{\mathring{A}}$


• a $\ce{C(sp)-C(sp^3)}$ single bond length is ~ 1.46 $\mathrm{\mathring{A}}$

In an earlier SE Chem post, the structure and bonding in the trigonal bipyramid molecule $\ce{PCl_5}$ was discussed. The molecular hybridization is pictured below. The axial bonds are hypervalent (this concept is explained in the earlier post) and they are constructed from p orbitals. The equatorial bonds are constructed from $\ce{sp^2}$ orbitals. General Rule #2 suggests that the equatorial bonds will be shorter than the axial bonds since the equatorial bonds contain more s-character.

As the following structural diagram shows, this is indeed the case.

Now let's turn our attention to the pentagonal bipyramid structure as found in $\ce{IF_7}$ for example. Here is the structure for this molecule

As you noted in your question, the equatorial bonds are now longer than the axial bonds. Based on what we've discussed thus far, we might make an initial "guess" that in $\ce{IF_7}$ the axial bonds have more s-character than the equatorial bonds. As it turns out, this is correct.

Here is a link to an article discussing the structure and bonding in $\ce{IF_7}$. Only the abstract is freely available. For completeness, I'll mention that there was an earlier SE Chem question related to $\ce{IF_7}$, but it doesn't seem to add much to the conversation. Returning to the published abstract, the key part of the abstract states,

"These features can be explained, however, by a bonding scheme involving a planar, delocalized pxy hybrid on the central atom for the formation of five equatorial, semi-ionic, 6-center 10-electron bonds and an sp hybrid for the formation of two mainly covalent axial bonds."

The abstract tells us that the equatorial bonds are formed from the $\ce{p_{x}}$ and $\ce{p_{y}}$ orbitals; the axial bonds are formed from the $\ce{sp_{z}}$ hybrid orbitals (after using two p orbitals for equatorial bonding we are left with one $\ce{p_{z}}$ orbital and one s orbital on the central iodine, hence, when they combine two sp hybridized orbitals result). That is the answer to the question. As we guessed, there is more s-character in the axial bonds than the equatorial bonds. Hence, the axial bonds are now shorter than the equatorial bonds.

For completeness

Someone, after reading that last paragraph, is going to notice that there are 5 equatorial fluorines, but only 4 bonding sites with the two p orbitals - what gives? First off, note that the $\ce{p_{x}}$ and $\ce{p_{y}}$ orbitals on iodine form hypervalent bonds with the p orbital on fluorine and so yes, there are only 4 equatorial points of attachment. But note that the abstract also mentioned that the equatorial bonds are semi-ionic. This suggests that we have a resonance structure where we have an [$\ce{IF_{6}^{+}}$][$\ce{F^{-}}$] ionic contributor. We can draw 4 more resonance structures just like this for the other 4 equatorial $\ce{I-F}$ bonds. This means that mixed in with our hypervalent p-p bonds is some ionic character - this allows us to make 5 equivalent equatorial bonds. Said differently, our 5 equatorial bonds are all identical resonance hybrids obtained by mixing 2 hypervalent p bonds (each one providing bonding for 2 fluorines) with 1 ionic bond (providing bonding for 1 fluorine).

parshanut torah comment - Inconsistency in listing creeping things in Noach's Ark's population

The animal population of Noach's Ark is commanded/listed six times in Parashat Noach:

1. 
   

   Genesis 6:19-20 (Initial command)

   
   


2. 
   
   

   Genesis 7:2-3 (Reiteration of command)

   
   


3. 
   

   Genesis 7:8-9 (Entry to the Ark)

   
   


4. 
   

   Genesis 7:14-16 (Reiteration of entry)

   
   


5. 
   

   Genesis 8:17 (Command to exit)

   
   
   


6. 
   

   Genesis 8:19 (Exit)

   
   

Each of these lists include animals, birds, and creeping things, except the second, which omits creeping things. Why?

Answer

The second one which does not mention creeping things is dividing the types into pure and impure, where pure animals and pure birds were brought in pairs of seven, and impure animals and impure birds were brought in twos. Creeping things were not so divided. Although there are pure creeping things none are appropriate (or really practical) as sacrifices which was the point of the extra amounts of pure fauna.

trop cantillation - How to read a mahpach psik?

I know that a mahpach not followed by a pashto is called a yetiv and has a different sound.

What about mahpach psik pashto, as in Shmuel Alef 19:9?

Would the mahpach there be read as connected to the pashto, and as such is a mahpach, or is it disconnected, and is a yetiv?

Answer

Let me explain a bit about the Yetiv.

A Yetiv is grammatically equivalent to a Pashta (3rd order disjunctive like Revi'i). It replaces Pashta on words with the accent on the opening syllable which do not have any assistants (words immediately prior with conjunctive accents). (This latter condition is similar to how Zakef Katan becomes Zakef Gadol and Segol becomes Shalshelet.)

Its symbol is the same as Mahpach's and it can be spotted by its placement just prior to the word it's on. (This is like how Pashta and Kadma have the same symbol but are distinguished by placement.)

You don't usually see a Yetiv preceding a Pashta because when in a sequence of adjacent Pashtas (ie. 3rd order disjuntcs) the first usually converts into a Revi'i. There are two instances in Torah (Vayikra 5:2 and Devarim 1:4) where there would be three Pashtas in a row, with the middle one being accented on its opening syllable. In this case, the first one becomes a Revi'i as usual and the second becomes a Yetiv as usual, resulting in the unusual formation of Yetiv followed by Pashta (confusing, as it looks remarkably like a standard Mahpach-Pashta combo).

There is no reason a Pesik should be relevant to these situations, so your case is a regular Mahpach-Pashta with a small pause in between.

orthography - Why was "アオ" used instead of ”あお”?

I was reading a picture book, and there was this sentence: アオくて　ちいさな　ぼくのイス。

Why is "ao" and "isu" written in katakana and not hiragana?

Answer

It's done because it makes it easier to read and understand when no kanji are used.

[青]{あお}くて[小]{ちい}さな[僕]{ぼく}の[椅子]{いす}。

アオくて　ちいさな　ぼくのイス。

あおくて　ちいさな　ぼくのいす。

あおくてちいさなぼくのいす。

あおくてち　いさなぼ　くのいす。

organic chemistry - Is benzene harmful to human health?

I have heard that benzene is a dangerous chemical for humans; it causes cancer and so it's use is restricted/regulated. But many organic compounds that we use contain benzene in some form such as oils, foods, even medicines (paracetomol). So is benzene really harmful to people's health?

Answer

As Ivan Neretin commented, parts of a compound can never dictate its own properties to the entire molecule's properties. Additionally benzene is not a very effective functional group by itself.

In short, a restricted use is only applied when that certain chemical is exposed by itself. In paracetamol or any other thing including benzene, you will not see any benzene vapor emanating from them or something. It would also be meaningful if benzene containing compounds degradation products included benzene, like in environmental degradation of chlorobenzenes, though this is not a usual case as I see.

halacha - Make for yourself a website?

Can a site like mi.yodeya.com be used for one to fulfill our sages mandate of "Make for yourself a Rav"?

Obviously I meant this facetiously. Of course a website cannot take the place of a rav, but did you see some of the halachic questions that are being asked and sound like they are being asked for practical reasons?!

Answer

This is an excellent and very, very, important question for a website like this, one that I've been thinking about from the beginning.

First of all, let me identify with the answers provided by Shalom and Alex and reiterate that mi.yodeya does not provide professional (particularly rabbinic) advice.(1) We try to alert users to this with red writing at the top of each page, a bold "Consult your Rabbi" suggestion on the "Ask a Question" page,(2) a disclaimer that comes up in red on the sidebar for new users and in bold in the the FAQ. In addition, I encourage answer-writers to stick in some humble language like "Here's my understanding, but ask your Rabbi before taking action." as necessary.

The remaining question is: What if people ignore the warnings and use mi.yodeya information as if it was professional rabbinic advice? I don't have an ironclad answer, but I think that on balance, we're doing OK. People give classes on Jewish law and practice all the time, and there's always the danger that attendees (or archive-listeners) will act on what they hear in the class or in an associated Q&A session without asking their Rabbi first.(3) The danger here is similar and, I think, similarly tolerable, with the advantage that we're explicitly warning against such practice at every turn.

Hopefully, more people will be encouraged to ask their Rabbi questions that otherwise wouldn't have than the other way around, thanks to our questions and answers spreading awareness of the issues and our disclaimers reminding people to go talk to their Rabbi.

All that said, I do need to upgrade my efforts (non-zero but not successful) to get Rabbinic guidance myself on this very point. And, of course, I'm very interested to hear what community members think.

UPDATE: I spoke to an internationally-respected rabbi who has experience and interest in Internet issues. He generally approved of the approach we're already taking, provided that we make sure that the disclaimers are prominent enough to be seen by everyone, and that we make it clear that while discussions of Jewish life naturally include Halachic content, people should go to a rabbi, not a website, for actual Halachic advice (my paraphrasing). I think we're pretty much fulfilling his advice already, but I'm going to review our disclaimers and consider beefing them up. Also, though the rabbi didn't bring it up, I'm seriously considering implementing some sort of flagging mechanism like the one Barry suggested. (See also my comment there.)

UPDATE 2 (2011 June 19 - after the migration to SE 2.0): When we migrated from the SE 1.0 site mi.yodeya, which I could edit more of, to this SE 2.0 site, the disclaimer went away, since it's not a standard part of an SE 2.0 site. We asked to have it put back, and SEI added it to the sidebar for new users. I also added it to the FAQ.

UPDATE 3 (2016 January 28 / 19 Shevat 5776): We now, once again, have a disclaimer on the top-right of most paged on Mi Yodeya. The same or similar text was already in our Tour, Help Center, and halacha tag rollover text.

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1) If you're wondering what it's good for, given that, please see my blog post on the topic.

2) See Update 2, above.

3) Recall the Blue Fringe lyric "I just heard a half hour halacha shiur / And decide to change the way I've lived for 18 years."