2. Listen to two ways of giving presentations and see if you can hear some of the phrases above.

3. Read some advices on delivering effective presentations in the Appendix 7 and write your own presentation for the topic “Molecular symmetry”.

Writing

Complaint letter

Look at Business English section13 and write a complaint email and the answer to it. You can use the email in exercise as a model. Use your own ideas or the ideas below.

You were delivered: PerkinElmerLuminescenceSpectrometrModel LS-50B

Thescanningrangeis 200-900 nm. Gatedphotomultiplierwithmodified S5 responseforoperationtoabout 650 nm. Interchangeable red-sensitivephotomultiplieravailableforuseto 900 nm. WavelengthAccuracyisplusorminus 1.0nm.

You ordered: Beckman du64 uv/VisSpectrophotometer

BeckmanModel DU64 UV/VisSpectrophotometer, Range: 700 to 250 nm, at 500 nm/mininabsorbancemode: 0.0 to 1.5 absorbanceunits.

Dear Ms. Martinez,
1 am writing in reference to the delivery of MP3 players, order # J396, which we received today.
Unfortunately, you sent us the wrong quantity. You sent us 20 K60 MP3 players, but we ordered 30. I have attached a copy of the order form for your reference.
I would be grateful if you would send ten more players as soon as possible.
Thank you for your assistance in this matter.
Sincerely,
David Heller
Dear Mr. Heller,
Thank you for your email concerning your order # J396.
We are sorry to hear that you received the wrong quantity. We have discovered that this was caused by a software problem. We will send you the remaining 10 MP3 players immediately, free of delivery charge.
Please accept our apologies for the inconvenience.
We look forward to doing business with you again in the future.
Sincerely,
Suzanne Martinez

Unit 4 stereochemistry of reactions

Reading

Section A

1. You are going to read the texts about Stereochemistry. Scan the texts and find the definitions of the following words:

optical activity,chiral,achiral,plane of symmetry,enantiomers,Fischer projections,diastereomers,meso compounds,racemic mixtures

When a beam of plane-polarized light is passed through a solution of certain organic molecules, the plane of polarization is rotated. Compounds that exhibit this behavior are called optically active. Optical activity is due to the asymmetric structure of the molecules themselves.

An object or molecule that is not superimposable on its mirror image is said to be chiral, meaning "handed". For example, a glove is chiral but a coffee cup is nonchiral, or achiral. A chiral molecule is one that does not contain a plane of symmetry - an imaginary plane that cuts through the molecule so that one half is a mirror image of the other half. The most common cause of chirality in organic molecules is the presence of a tetrahedral, sp³-hybridized carbon atom bonded to four different groups. Compounds that contain such chiral carbon atoms exist as a pair of non-superimposable, mirror-image stereoisomers called enantiomers. Enantiomers are identical in all physical properties except for the direction in which they rotate plane-polarized light.

The stereochemical configuration of a chiral carbon atom can be depicted using Fischer projections, in which horizontal lines (bonds) are understood to come out of the plane of the paper and vertical bonds are understood to go back into the plane of the paper. The configuration can be specified as either R (rectus) or S (sinister) by using the Cahn-Ingold-Preloe sequence rules. This is done by first assigning priorities to the four substituents on the chiral carbon atom and then orienting the molecule so that the lowest-priority group points directly back away from the viewer. We then look at the remaining three substituents and let the eye travel from the group having the highest priority to second highest to third highest. If the direction of travel is clockwise, the configuration is labeled R; if the direction of travel is counterclockwise, the configuration is labeled S.

Some molecules possess more than one chiral center. Enantiomers have opposite configuration at all chiral centers, whereas diastereomers have the same configuration in at least one center but opposite configurations at the others. Epimers are diastereomers that differ in configuration at onlyone chirality center. A compound with n chiral centers can have 2ⁿ stereoisomers.

Meso compounds contain chiral centers, but are achiral overall because they contain a plane of symmetry. Racemic mixtures, or race-mates are 50:50 mixtures of ( + ) and (-) enantiomers. Racemic mixtures and individual diastereomers differ from each other in their physical properties such as solubility, melting point, and boiling point.

Most reactions give chiral products. If the starting materials are optically inactive, the products must also be optically inactive—either meso or racemic. If one or both of the starting materials is optically active, however, the product will also be optically active if the original chiral center remains.

A molecule is prochiral if can be converted from achiral to chiral in a single chemical step. A prochiral sp²-hybridized atom has two faces, described as either Re or Si. An sp^3-hybridized atom is a prochirality center if, by changing one of its attached atoms, a chirality center results. The atom whose replacement leads to an R chirality center is pro-R, and the atom whose replacement leads to an S chirality center is pro-S.

2. There are some more terms dealing with stereochemistry, try to explain them:

plane-polarized light, stereochemical configuration, absolute configuration, chiral environment, chirality center, configuration, dextrorotatory, epimers, levorotatory, pro-R configuration, pro-S configuration, prochiral, prochirality center, Re face, resolution, S configuration, Siface, specific rotation.

3. Read the text again, divide it into logical parts and write possible titles for them.

4. Choose any part of the text and present it as a separate topic. Give as much information on it as possible.

5. Render the text.

Section B

1. The title of the text in this section is Chiral molecules with two stereogenic centers. What is the likely content of the article?

2. You are going to read a text about chiral molecules. Four paragraphs have been removed from the text. Choose from the paragraphs A-E the one which fits each gap (1-4). There is one extra paragraph which you do not need to use.

When a molecule contains two stereogenic centers, as does 2,3-dihydroxybutanoic acid, how many stereoisomers are possible?

1__________________________________________________________

The four possible combinations of these two stereogenic centers are (2R,3R) (stereoisomer I ); (2S,3S)(stereoisomer II); (2R,3S)(stereoisomer III); (2S,3R) (stereoisomer IV).

Stereoisomer I is not a mirror image of III or IV, so is not an enantiomer of either one. Stereoisomers that are not related as an object and its mirror image are called diastereomers; diastereomers are stereoisomers that are not enantiomers. Thus, stereoisomer I is a diastereomer of III and a diastereomer of IV. Similarly, II is a diastereomer of III and IV.

2____________________________________________________________

Enantiomers must have equal and opposite specific rotations. Diastereomeric sub­stances can have different rotations, with respect to both sign and magnitude. The (2R,3R)and (2S,3S)enantiomers (I and II) have specific rotations that are equal in magnitude but opposite in sign. The (2R,3S)and (2S,3R)enantiomers (III and IV) likewise have specific rotations that are equal to each other but opposite in sign. The magnitudes of rotation of I and II are different, however, from those of their diastereomers III and IV.

3___________________________________________________________

Organic chemists use an informal nomenclature system based on Fischer projec­tions to distinguish between diastereomers. When the carbon chain is vertical and like substituents are on the same side of the Fischer projection, the molecule is described as the erythrodiastereomer. When alike substituents are on opposite sides of the Fischer projection, the molecule is described as the threo diastereomer.

4___________________________________________________________

The situation is the same when the two stereogenic centers are present in a ring. There are four stereoisomers I-bromo-2-chlorocyclopropanes: a pair of enantiomers in which the halogens are trans and a pair in which they are cis. The cis compounds are diastereomers of the trans.

In writing Fischer projections of molecules with two stereogenic centers, the mol­ecule is arranged in an eclipsedconformation for projection onto the page. Again, horizontal lines in the projection represent bonds coming toward you; vertical bonds point away.

We can use straightforward reasoning to come up with the answer. The absolute config­uration at C-2may be R or S. Likewise, C-3 may have either the R or the Sconfigura­tion.

Because diastereomers are not mirror images of each other, they can have quite different physical and chemical properties. For example, the (2R,3R)stereoisomer of 3-amino-2-butanol is a liquid, but the (2R,3S)diastereomer is a crystalline solid.

When using Fischer projections for this purpose, however, be sure to remember what three-dimensional objects they stand for. One should not, for example, test for superpo­sition of the two chiral stereoisomers by a procedure that involves moving any part of a Fischer projection out of the plane of the paper in any step.

To convert a molecule with two stereogenic centers to its enantiomer, the config­uration at both centers must be changed. Reversing the configuration at only one stereo­genic center converts it to a diastereomeric structure.

3. Build a step-by-step picture of the text using a Flow Chart. Look at Appendix 4-6 for help.

4. Using your Flow Chart retell the text.

5. Give any additional information on the topic.

Speaking

1. EVASIVE PARTNERS: There is an article about chiral drugs presented below. Work in pairs. Imagine thatyou are representative of the British pharmaceutical company produced thalidomide that caused over 2000 cases of seri­ous birth defects in children born to women who took it while pregnant. You are giving an interview.You area little evasive and provide one- or two-word replies to questions. Interviewer has to dig deeper in trying to find out more about the causes of the incident. Use the following exponents to get more information:

• Could you tell me more?

• Would you like to expand on that?

• Could you add to that?

• That only answers half the question.

• You’re being a little evasive here. Could you add more?

• A little more explanation would be nice.