What is the approximate H-C-H bond angle in methane, CH4?
109.5 degrees
✓
120 degrees
180 degrees
90 degrees
Correct Answer
109.5 degrees
The carbon in methane is sp3 hybridized, with four bonds pointing to the corners of a tetrahedron at about 109.5 degrees. An angle of 120 degrees belongs to trigonal planar sp2 carbons, as in ethene. An angle of 180 degrees belongs to linear sp carbons, as in ethyne. An angle of 90 degrees is not the angle of an unstrained tetrahedral carbon such as the one in methane.
Category: Alkanes
Question 2
Which general formula describes the open-chain alkanes?
CnH2n
CnH2n+2
✓
CnH2n-2
CnHn
Correct Answer
CnH2n+2
Every carbon in a chain alkane forms single bonds, and the two end carbons each carry one extra hydrogen, giving CnH2n+2, as in propane, C3H8. CnH2n fits alkenes with one double bond and also cycloalkanes. CnH2n-2 fits alkynes with one triple bond. CnHn fits a few special molecules such as benzene, C6H6, not alkanes.
Category: Alkanes
Question 3
What is the correct IUPAC name for CH3CH(CH3)CH2CH3?
3-methylbutane
2-ethylpropane
2-methylbutane
✓
1,1-dimethylpropane
Correct Answer
2-methylbutane
The longest chain has four carbons, so the parent is butane, and the chain is numbered from the end nearer the branch, putting the methyl group on carbon 2. 3-methylbutane numbers the chain from the wrong end. 2-ethylpropane fails to use the longest chain. 1,1-dimethylpropane also chooses a chain that is too short.
Category: Alkanes
Question 4
Ethene reacts with bromine, Br2, to form 1,2-dibromoethane. Which type of reaction is this?
Substitution
Elimination
Combustion
Addition
✓
Correct Answer
Addition
In an addition reaction, atoms add across a double bond and the pi bond is broken, so each carbon of ethene gains one bromine atom. Substitution replaces one atom with another and is typical of alkanes. Elimination removes atoms to form a double bond, the reverse of this change. Combustion is a reaction with oxygen that produces carbon dioxide and water.
Category: Alkenes and Alkynes
Question 5
How many different structural isomers have the molecular formula C5H12?
3
✓
2
4
5
Correct Answer
3
C5H12 has three structural isomers: pentane, 2-methylbutane and 2,2-dimethylpropane. An answer of 2 is the number of isomers of C4H10. An answer of 4 usually comes from counting one structure twice under two names. An answer of 5 confuses the number of carbon atoms with the number of isomers.
Category: Alkanes
Question 6
Which compound can exist as separate cis and trans isomers?
1-butene
2-butene
✓
Propene
2-methylpropene
Correct Answer
2-butene
In 2-butene each carbon of the double bond carries one hydrogen and one methyl group, so the methyl groups can be on the same side or opposite sides. 1-butene has two hydrogens on one end of the double bond, so no cis or trans form exists. Propene also has two hydrogens on one double-bond carbon. 2-methylpropene has two methyl groups on the same carbon, which rules out cis and trans forms.
Category: Alkenes and Alkynes
Question 7
Which hydrocarbon contains a carbon-carbon triple bond?
Ethene
Ethane
Ethyne
✓
Benzene
Correct Answer
Ethyne
Names ending in -yne indicate an alkyne, a hydrocarbon with a triple bond; ethyne, HCCH, has a triple bond between its two carbons. Ethene contains a double bond. Ethane contains only single bonds. Benzene is an aromatic ring with delocalized pi electrons, not a triple bond.
Category: Alkenes and Alkynes
Question 8
Benzene is far less reactive toward bromine than ethene is. What explains this difference?
It contains only single carbon-carbon bonds
Its carbon atoms are sp3 hybridized
It has no hydrogen atoms to replace
Its pi electrons are delocalized around the ring
✓
Correct Answer
Its pi electrons are delocalized around the ring
The six pi electrons of benzene are spread evenly over the whole ring, which makes the ring especially stable and resistant to reactions that would break up that delocalization. Benzene does not contain only single carbon-carbon bonds; its bonds are intermediate between single and double. Its carbon atoms are sp2 hybridized, not sp3. Benzene has six hydrogen atoms, one on each carbon.
Category: Aromatic Compounds
Question 9
Which functional group defines an alcohol?
Hydroxyl group
✓
Carbonyl group
Carboxyl group
Amino group
Correct Answer
Hydroxyl group
Alcohols contain an -OH hydroxyl group bonded to a carbon atom, as in ethanol, CH3CH2OH. A carbonyl group, C=O, defines aldehydes and ketones. A carboxyl group, -COOH, defines carboxylic acids. An amino group, -NH2, is found in amines.
Category: Alcohols and Ethers
Question 10
An oxygen atom bonded to two carbon-containing groups, R-O-R', is characteristic of which class of compounds?
Alcohols
Ethers
✓
Ketones
Esters
Correct Answer
Ethers
Ethers have an oxygen atom single bonded to two carbon groups, as in diethyl ether. Alcohols have oxygen bonded to one carbon group and one hydrogen. Ketones have a carbon double bonded to oxygen. Esters contain a carbonyl group whose carbon is also bonded to an -O-R group.
Category: Alcohols and Ethers
Question 11
Oxidizing an alcohol whose -OH carbon is bonded to two other carbon atoms, such as 2-butanol, produces which kind of compound?
Aldehyde
Carboxylic acid
Ketone
✓
Ether
Correct Answer
Ketone
Oxidation replaces the C-H bond on the -OH carbon with a C=O bond; when that carbon is bonded to two other carbons, the product is a ketone, such as butanone from 2-butanol. An aldehyde forms when the -OH is on an end carbon. A carboxylic acid forms by further oxidation of an aldehyde. An ether is not produced by oxidizing an alcohol.
Category: Carbonyl Compounds
Question 12
Heating acetic acid with ethanol in the presence of an acid catalyst produces water and which other compound?
Methyl acetate
Diethyl ether
Acetone
Ethyl acetate
✓
Correct Answer
Ethyl acetate
A carboxylic acid and an alcohol combine to form an ester and water; the ethyl group comes from ethanol and the acetate part from acetic acid, giving ethyl acetate. Methyl acetate would require methanol. Diethyl ether forms from two ethanol molecules, not from an acid and an alcohol. Acetone is a ketone and is not formed in this reaction.
Category: Carbonyl Compounds
Question 13
Which structural feature identifies a compound as an aldehyde rather than another type of C=O compound?
The C=O carbon is bonded to at least one hydrogen
✓
The C=O carbon is bonded to two carbon groups
The C=O carbon is also bonded to an -OH group
The C=O carbon is also bonded to a nitrogen
Correct Answer
The C=O carbon is bonded to at least one hydrogen
In an aldehyde the carbonyl carbon carries at least one hydrogen atom, which is why aldehydes are found at the end of a carbon chain. A carbonyl carbon bonded to two carbon groups is a ketone. A carbonyl carbon also bonded to an -OH group is a carboxylic acid. A carbonyl carbon also bonded to a nitrogen is an amide.
Category: Carbonyl Compounds
Question 14
Amines behave as weak bases. Which feature of the nitrogen atom explains this?
It can donate a proton to water
It has a lone pair that can accept a proton
✓
It has an empty orbital that accepts electrons
It carries a permanent positive charge
Correct Answer
It has a lone pair that can accept a proton
Like ammonia, an amine has a lone pair of electrons on nitrogen that can bond to H+, forming an ammonium ion. Donating a proton to water would make the amine an acid, not a base. An empty orbital that accepts electrons describes a Lewis acid. A neutral amine carries no permanent positive charge; it becomes positive only after accepting a proton.
Category: Amines and Amides
Question 15
How is the amine (CH3)2NH classified?
A primary amine
A tertiary amine
A secondary amine
✓
A quaternary ammonium ion
Correct Answer
A secondary amine
Dimethylamine has two carbon groups and one hydrogen on its nitrogen, which makes it a secondary amine. A primary amine has only one carbon group on nitrogen, as in methylamine. A tertiary amine has three carbon groups and no N-H bond. A quaternary ammonium ion has four carbon groups and a positive charge.
Category: Amines and Amides
Question 16
Reacting a carboxylic acid with an amine, with the loss of water, forms which functional group?
Ester
Ether
Ketone group
Amide
✓
Correct Answer
Amide
In an amide the carbonyl carbon is bonded to nitrogen; this is the same linkage that joins amino acids in proteins. An ester forms when a carboxylic acid reacts with an alcohol instead. An ether has oxygen between two carbon groups and no carbonyl. A ketone group has a carbonyl carbon bonded to two carbons, not to nitrogen.
Category: Amines and Amides
Question 17
Ethanol, CH3CH2OH, and dimethyl ether, CH3OCH3, have the same molecular formula, yet ethanol boils at a much higher temperature. Why?
Ethanol molecules hydrogen bond to one another
✓
Ethanol has a larger molar mass
Dimethyl ether is an ionic compound
Ethanol molecules contain more electrons
Correct Answer
Ethanol molecules hydrogen bond to one another
The O-H group of ethanol lets its molecules form hydrogen bonds with each other, which takes extra energy to overcome; dimethyl ether has no O-H bond. Ethanol does not have a larger molar mass, since both are C2H6O. Dimethyl ether is a covalent molecular compound, not ionic. The two molecules contain exactly the same number of electrons.
Category: Alcohols and Ethers
Question 18
What is the electron-pair geometry around the carbon atom of a carbonyl group?
Tetrahedral
Trigonal planar
✓
Linear
Trigonal pyramidal
Correct Answer
Trigonal planar
The carbonyl carbon is sp2 hybridized and has three groups around it, giving a flat arrangement with angles of about 120 degrees. Tetrahedral applies to sp3 carbons with four single bonds. Linear applies to sp carbons such as those in a triple bond. Trigonal pyramidal requires a lone pair on the central atom, which a carbonyl carbon does not have.
Category: Carbonyl Compounds
Question 19
Which compound is a structural isomer of 1-propanol, CH3CH2CH2OH?
Propanal
Propanoic acid
2-propanol
✓
Propene
Correct Answer
2-propanol
2-propanol, CH3CH(OH)CH3, has the same formula as 1-propanol, C3H8O, but the -OH is on the middle carbon. Propanal is C3H6O, with two fewer hydrogen atoms. Propanoic acid is C3H6O2. Propene is C3H6 and contains no oxygen.
Category: Alcohols and Ethers
Question 20
A hydrocarbon whose carbon atoms are joined only by single bonds is described as which?
Unsaturated
Aromatic
Conjugated
Saturated
✓
Correct Answer
Saturated
A saturated hydrocarbon holds the largest possible number of hydrogen atoms because it has no double or triple bonds; alkanes are saturated. Unsaturated hydrocarbons contain at least one double or triple bond. Aromatic compounds contain a benzene-type ring of delocalized pi electrons. Conjugated describes alternating single and multiple bonds.
Category: Alkanes