CHEM 105 GOB Practice Exam 2 (Ch. 14 & 15) – Step-by-Step Study Guidance

Q1. Draw the skeletal ("line") structure of 1,5-dihydroxy-4-methyl-2-pentanone.

Background

Topic: Organic Nomenclature and Structure Drawing

This question tests your ability to interpret IUPAC names and translate them into skeletal (line) structures, focusing on functional groups and substituent positions.

Key Terms and Concepts:

• Skeletal (line) structure: A simplified organic structure where carbon atoms are represented by line ends and vertices, and hydrogens attached to carbons are usually omitted.

• 1,5-dihydroxy: Hydroxyl (-OH) groups on carbons 1 and 5.

• 4-methyl: A methyl (-CH3) group on carbon 4.

• 2-pentanone: A five-carbon chain with a ketone (C=O) on carbon 2.

Step-by-Step Guidance

1. Draw a five-carbon chain to represent pentane.

2. Number the carbons from left to right (or right to left), ensuring you can place the functional groups correctly.

3. Place a ketone group (C=O) on carbon 2.

4. Add hydroxyl (-OH) groups to carbons 1 and 5.

5. Add a methyl group (-CH3) to carbon 4.

Try drawing the structure before checking the answer!

Q2. Write the common (not systematic) name of each organic molecule.

Background

Topic: Common Names of Organic Compounds

This question tests your ability to recognize and recall the common (trivial) names of organic molecules, which often differ from their systematic IUPAC names.

Key Terms and Concepts:

• Common name: The traditional name for a molecule, often based on historical or source-based naming rather than IUPAC rules.

• Examples include names like acetone, formaldehyde, acetic acid, etc.

Step-by-Step Guidance

1. Examine the structure provided and identify the functional group(s) present (e.g., aldehyde, ketone, carboxylic acid).

2. Count the number of carbons in the main chain to help match the structure to a common name.

3. Recall the common names associated with that functional group and carbon count.

4. Double-check for any substituents or branching that might affect the common name.

Try matching the structure to a common name before revealing the answer!

Q3. Predict the product of this organic reaction: CH3–CH=CH–CO–CH3 + H2 (Pt catalyst). Draw the condensed structure of P.

Background

Topic: Hydrogenation of Alkenes and Ketones

This question tests your understanding of catalytic hydrogenation, where H2 is added across double bonds (and sometimes carbonyls) in the presence of a metal catalyst like Pt.

Key Terms and Concepts:

• Hydrogenation: The addition of hydrogen (H2) to unsaturated bonds (C=C or C=O) using a catalyst.

• Condensed structure: A way of writing organic molecules that shows all atoms but groups them to save space.

Step-by-Step Guidance

1. Identify all double bonds in the starting molecule (C=C and C=O).

2. Determine which bonds are reduced under catalytic hydrogenation (typically C=C first, then possibly C=O).

3. Add H2 across the reducible double bond(s), converting them to single bonds.

4. Redraw the molecule in condensed form, showing the new single bonds and added hydrogens.

Try drawing the product before checking the answer!

Q4. Determine whether the following molecule is a hemiacetal, acetal, or neither. Highlight the relevant carbon if present.

Background

Topic: Functional Groups – Hemiacetals and Acetals

This question tests your ability to recognize hemiacetal and acetal functional groups in organic molecules.

Key Terms and Concepts:

• Hemiacetal: A carbon bonded to one –OH group and one –OR group (plus two other groups).

• Acetal: A carbon bonded to two –OR groups (plus two other groups).

Step-by-Step Guidance

1. Locate the carbon atom(s) attached to oxygen atoms in the structure.

2. Check if any carbon is bonded to both an –OH and an –OR group (hemiacetal) or to two –OR groups (acetal).

3. If neither pattern is present, the molecule is neither a hemiacetal nor an acetal.

4. Highlight the relevant carbon if you find a hemiacetal or acetal center.

Try classifying the molecule before checking the answer!

Q5. Check the box under each aldose.

Background

Topic: Carbohydrate Classification

This question tests your ability to distinguish aldoses (sugars with an aldehyde group) from other monosaccharides.

Key Terms and Concepts:

• Aldose: A monosaccharide with an aldehyde functional group at carbon 1.

• Ketose: A monosaccharide with a ketone group, usually at carbon 2.

Step-by-Step Guidance

1. Examine each sugar structure for the presence of an aldehyde group (–CHO) at the top (carbon 1).

2. If the structure has a carbonyl at carbon 1, it is an aldose; if at carbon 2, it is a ketose.

3. Check the appropriate box for each aldose you identify.

Try identifying the aldoses before checking the answer!

Q6. Check the box under each compound that exists as a pair of mirror-image twins (enantiomers).

Background

Topic: Stereochemistry – Enantiomers

This question tests your ability to recognize molecules that have non-superimposable mirror images (enantiomers).

Key Terms and Concepts:

• Enantiomers: Stereoisomers that are non-superimposable mirror images of each other.

• Chirality center: A carbon atom bonded to four different groups.

Step-by-Step Guidance

1. Identify any chiral centers in each molecule (carbons with four different groups attached).

2. Determine if the molecule has a non-superimposable mirror image (i.e., if it is chiral).

3. If so, check the box indicating it exists as a pair of enantiomers.

Try identifying the enantiomers before checking the answer!

Q7. Are the monosaccharides below enantiomers? If not, edit either molecule so that they are enantiomers.

Background

Topic: Stereochemistry of Monosaccharides

This question tests your understanding of Fischer projections and how to recognize or create enantiomeric pairs.

Key Terms and Concepts:

• Fischer projection: A two-dimensional representation of a three-dimensional organic molecule, commonly used for sugars.

• Enantiomers: Molecules that are mirror images but not superimposable.

Step-by-Step Guidance

1. Compare the configuration (left/right positions of –OH and –H) at each chiral center in both molecules.

2. If all chiral centers are opposite, the molecules are enantiomers; if not, identify which centers need to be changed.

3. Edit the Fischer projection of one molecule so that all chiral centers are opposite to the other.

Try editing the projections before checking the answer!

Q8. Draw a Haworth projection of a common cyclic form of this monosaccharide.

Background

Topic: Carbohydrate Cyclization – Haworth Projections

This question tests your ability to convert a Fischer projection of a monosaccharide into its cyclic (Haworth) form.

Key Terms and Concepts:

• Haworth projection: A way to represent the cyclic structure of sugars, showing the ring and substituents above or below the plane.

• Anomeric carbon: The new stereocenter formed during cyclization (usually carbon 1 in aldoses).

Step-by-Step Guidance

1. Identify the number of carbons and the positions of –OH groups in the Fischer projection.

2. Determine which –OH attacks the carbonyl carbon to form the ring (usually C5 –OH attacks C1 in hexoses).

3. Draw the ring, placing substituents above or below the plane according to the Fischer projection.

Try drawing the Haworth projection before checking the answer!

Q9. Draw a Fischer projection of the molecule that would produce L-threitol if it were reduced.

Background

Topic: Sugar Reduction and Fischer Projections

This question tests your understanding of how reduction (conversion of carbonyl to alcohol) affects the structure of monosaccharides.

Key Terms and Concepts:

• Reduction: The process of converting an aldehyde or ketone group to an alcohol.

• Fischer projection: A two-dimensional representation of a sugar's stereochemistry.

Step-by-Step Guidance

1. Recall that L-threitol is the reduced form of L-threose (an aldose).

2. Draw the Fischer projection of L-threose, noting the configuration of –OH groups.

3. Ensure the carbonyl group is at the top (C1) and the rest of the chain matches L-threose's stereochemistry.

Try drawing the Fischer projection before checking the answer!

Q10. Write the systematic name of each organic molecule.

Background

Topic: IUPAC Nomenclature

This question tests your ability to apply IUPAC rules to name organic molecules based on their structure.

Key Terms and Concepts:

• Systematic (IUPAC) name: The official name of a compound based on the longest carbon chain, functional groups, and substituents.

• Identify the parent chain, number the carbons, and assign locants to substituents and functional groups.

Step-by-Step Guidance

1. Identify the longest continuous carbon chain containing the highest-priority functional group.

2. Number the chain to give the functional group the lowest possible number.

3. Name and number all substituents.

4. Assemble the name in the correct IUPAC order.

Try naming the molecules before checking the answer!

Q11. Is this a reducing sugar? Does this molecule contain a glycosidic bond? If so, write the symbol and the common names of the molecules released upon hydrolysis.

Background

Topic: Carbohydrate Chemistry – Reducing Sugars and Glycosidic Bonds

This question tests your ability to identify reducing sugars, recognize glycosidic bonds, and understand hydrolysis products.

Key Terms and Concepts:

• Reducing sugar: A sugar with a free anomeric carbon capable of acting as a reducing agent.

• Glycosidic bond: A covalent bond joining two monosaccharides via an oxygen atom.

• Anomeric carbon: The carbon derived from the carbonyl group during cyclization.

Step-by-Step Guidance

1. Examine the structure for a free anomeric carbon (not involved in a glycosidic bond) to determine if it is a reducing sugar.

2. Look for an oxygen bridge between two monosaccharide units to identify a glycosidic bond.

3. If a glycosidic bond is present, determine its position and orientation (e.g., α(1→4)).

4. Identify the monosaccharide units that would be released upon hydrolysis, including their anomeric and enantiomeric forms.

Try answering each part before checking the answer!

Q12. Check the box under each L-aldopentose.

Background

Topic: Carbohydrate Stereochemistry – L- and D- Sugars

This question tests your ability to recognize L-aldopentoses among various sugar structures.

Key Terms and Concepts:

• L-aldopentose: A five-carbon aldose (aldehyde sugar) with the configuration of the L-series (–OH on the left at the bottom chiral center in Fischer projection).

• Fischer projection: Used to distinguish D- and L- forms based on the position of the –OH group at the penultimate carbon.

Step-by-Step Guidance

1. Identify all five-carbon sugars with an aldehyde group at the top (aldopentoses).

2. Check the configuration at the bottom chiral center; if the –OH is on the left, it is an L-sugar.

3. Check the appropriate box for each L-aldopentose you find.

Try identifying the L-aldopentoses before checking the answer!