Who has never cried while cutting an onion?
This is a little explanation in easy terms.
Inside the onion cells there are some chemical compounds that contain sulfur. When you cut an onion its cells are broken and those chemical compounds then undergo a reaction that transforms them into a more volatile sulfured products, which are released into the air.
These sulfured compounds react with the moisture in your eyes forming sulfuric acid, which produces a burning sensation. The nerve endings in your eyes are very sensitive and so they pick up on this irritation. The brain reacts by telling your tear ducts to produce more water, to dilute the irritating acid. So you cry to keep your eyes protected from the acid.
There are some tricks to make onion-dicing less problematic:
- Chop the onion under cold water. The volatile sulfured compounds will be released but then they react with the water, instead of reaching your eyes.
- You can freeze the onion for 10 minutes before cutting it. The cold temperature of the onion will slow down the chemical reaction which forms the volatile sulfured compunds.
The Diels-Alder reaction is essentially a cycloaddition reaction ( Cycloaddition reaction is one example of a special class of reactions called pericyclic reaction. Other examples of pericyclic reactions are electrocyclic reactions, sigmatropic reactions ) in which an alkene adds to a 1,3-diene to form a 6-membered ring. The reaction is synthetically very useful, due to the formation of cyclic product
MECHANISM:
The Diels-Alder reaction mechanism still remains the topic of much debate because of the uncertainty as to whether the mechanism is stepwise, concerted, or free-radical. For the time being arrow-pushing concerted mechanism is preferred on order to better understand the beauty of this reaction.
Important Points to Remember:
1. The Diels-Alder reaction (a
4π+2π4π+2π size 12{"4π"+"2π"} {} cycloaddition) is a very important synthetic method for the preparation of cyclohexanes, mainly because the stereochemistries of the products are so well controlled.
2. Normal Diels-Alder reaction is favoured by electron withdrawing groups (EWG) on the electrophilic dienophile and by electron donating groups on the nucleophilic diene.
3. The diene must be cisoid (this refers to the conformation about the single bond). Example: trans-butene does not undergo DA reaction where cis-butene does.
4. The reaction is usually thermodynamically favored due to the conversion of
5. The Diels-Alder reaction is stereospecific with respect to both the diene and the dienophile.
6. Under the reaction conditions, the product can be broken down to its starting materials known as retro Diels-Alder reaction.
7. To predict the relative rates of reaction of dienes, use these rules.
(a) Only conjugated dienes react as dienes in Diels-Alder processes. Isolated and
cumulated dienes may react as dienophiles, but they cannot react as dienes.
(b) Dienes in the s-trans conformation do not react in Diels-Alder reactions. Acyclic
dienes may rotate around a single bond to become s-cis, but some dienes are locked in the
s-trans conformation in cyclic structures and cannot react.
(c) Dienes containing electron-releasing groups (usually alkyl groups) react faster,
especially with dienophiles that contain electron-withdrawing groups (almost always groups that contain electronegative atoms). The opposite combination also gives relatively fast reactions, but the starting materials are unusual.
The career options in chemistry are practically endless! However, your employment options depend on how far you have taken your education. A 2-year degree in chemistry won't get you very far. You could work in some labs washing glassware or assist at a school with lab preparation, but you wouldn't have much advancement potential and you could expect a high level of supervision. A college bachelor's degree in chemistry (B.A., B.S.) opens up more opportunities. A 4-year college degree can be used to gain admittance to advanced degree programs (e.g., graduate school, medical school, law school). With the bachelor's degree, you can get a bench job, which would allow you to run equipment and prepare chemicals. A bachelor's degree in chemistry or education (with a lot of chemistry) is necessary to teach at the K-12 level. A master's degree in chemistry, chemical engineering, or other field opens up far more options. A terminal degree, such as a Ph.D. or M.D., leaves the field wide open. In the United States you need at least 18 graduate credit hours to teach at the college level (preferably a Ph.D.). Most scientists who design and supervise their own research programs have terminal degrees. Chemistry is a part of biology and physics, plus, there are lots of categories of chemistry! Here's look at some of the career options related to chemistry:
Chemistry (chemist profile)
Ethnobotany
Environmental Law
Patent Law
Technical Writing
Pharmaceuticals
Oceanography
Software Design
Space Exploration
Government Policy
Forensic Science
Biotechnology
Metallurgy
Ceramics Industry
Plastics Industry
Paper Industry
Medicine
Teaching
Engineering (chemical engineer profile)
Geochemistry
Agrochemistry
Military Systems
This list isn't remotely complete. You can work chemistry into any industrial, educational, scientific, or governmental field. Chemistry is a very versatile science. Mastery of chemistry is associated with excellent analytical and mathematical skills. Students of chemistry are able to solve problems and think things through. These skills are useful for any job!
We're all chemists. We use chemicals every day and perform chemical reactions without thinking much about them. Chemistry is important because everything you do is chemistry! Even your body is made of chemicals. Chemical reactions occur when you breathe, eat, or just sit there reading. All matter is made of chemicals, so the importance of chemistry is that it's the study of everything.
Everyone can and should understand basic chemistry, but it may be important to take a course in chemistry or even make a career out of it. It's important to understand chemistry if you are studying any of the sciences because all of the sciences involve matter and the interactions between types of matter. Students wanting to become doctors, nurses, physicists, nutritionists, geologists, pharmacists, and (of course) chemists all study chemistry. You might want to make a career of chemistry because chemistry-related jobs are plentiful and high-paying. The importance of chemistry won't be diminished over time, so it will remain a promising career path.
