ochem lab final (TAMU 237)

After a ring flip, an axial substituent in one chair form becomes an equatorial substituent.

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Substituents larger than hydrogen prefer to adopt equatorial positions due largely to a smaller amount of steric strain.

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The different geometries that a molecule can attain by bond rotations and bends are called conformations.

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What is true about equatorial and axial positions in cyclohexane?

1. Each carbon in cyclohexane is attached to one axial and one equatorial hydrogen. 2. Each face of the chair conformation of cyclohexane ring has three axial and three equatorial hydrogens in an alternating pattern.

Which of the following statements are true of the chair conformation of cyclohexane?

1. It contains six equatorial hydrogens that radiate around ring equator. 2. It contains six axial hydrogens that are perpendicular to the ring.

Which of the following statements about melting point determination are true?

1. Pure substances typically have a melting range of less than 2 degrees Celsius. 2. The lower limit of the melting temperature range occurs when the first crystal starts to melt. 3. The upper limit of the melting temperature range occurs when the last remaining crystal melts.

When comparing melting temperature ranges of pure and impure samples, what is true?

1. The melting temperature range of an impure sample is broader than that of a pure sample. 2. The melting temperature range of an impure sample is lower than that of a pure sample.

In which of the following ways is the use of an infrared spectrophotometer fitted with an ATR sample advantageous over a beam-through-sample spectrophotometer?

1. The same ATR sample holder may be used for liquid samples and solid samples. 2. ATR allows for the use very small samples

Which of the following are requirements for a suitable solvent when performing a recrystallization?

1. The solvent should either dissolve the impurities at all temperatures or not dissolve the impurities at all. 2. The solvent should not dissolve the compound when cold.

During the developing of a TLC plate, it is common to place a cover on the chromatography chamber and have a piece of moist filter paper line the walls of the TLC chamber. Why is the moist filter paper in the TLC chamber of importance during thin-layer chromatography?

1. To ensure that the air in the chamber is saturated with solvent vapor. 2. To prevent evaporation from the TLC plate during development.

What region of an infrared spectrum is most useful for distinguishing between isomers that contain the same functional group?

1400-400 cm-1

Infrared Spectra are recorded using an ATR detector. What does ATR stand for?

Attenuated Total Reflectance

Why is it good practice to perform a mixed melting point determination at two different ratios of unknown to known materials in the melting point capillary?

Because it is possible that one particular combination of materials generates a eutectic.

1600 cm -1

C=C alkene

1800-1640 cm-1

C=O Ester

4000-3000 cm-1

O-H carboxylic acid

During the recrystallization of a solid, the sample is dissolved in hot solvent then filtered while hot. The filtrate is allowed to cool and the mixture is filtered a second time. What effect will result if the filtrate solution is cooled too rapidly?

Small impure crystals will form.

eutectic

The composition of a mixture of two components that generates the lowest possible melting point.

How would the melting range of a pure organic material be changed if there was a tiny shard of broken glass inside the melting point capillary?

The melting range would not be affected.

When is recrystallization not a suitable method to purify an organic material?

When the organic material is a liquid.

The Rf value is defined as the ratio between the distance the spot moved and the distance the solvent moved.

true