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Organic Chemistry
Chemistry Regents June 2006 - Question 71 PDF Print E-mail
chemjun06q71

Correct answer: This compound is a thiol because it contains the S-H functional group of thiols.

Why?:

A thiol is a sulfur analogue of an alcohol functional group (O-H). Instead of oxygen, it is replaced by a sulfur atom. Thiols are also referred to as mercaptans.

Answering this question:

It can be observed in the structure of the compound that it contains the S-H group attached to the 1st carbon. Therefore, this compound is a thiol.

So this is a thiol because it contains the S-H functional group of thiols.

 
Chemistry Regents June 2006 - Question 62 PDF Print E-mail
chemjun06q62

Why?:

The alkane C4H10 is a straight chain alkane. It is n-butane. The secondary hydrogen atoms are more reactive towards free radical substitution that the primary ones. It is expected that a chlorine atom in place of a secondary hydrogen atom is a major product.

Answering this question:

Draw first the structure for n-butane. Replace one hydrogen atom on the 2nd carbon by a chlorine atom. Since secondary hydrogen atoms are more reactive, this would be the major product. Then replace one of the hydrogen atoms in the first carbon by a chlorine atom. This would be the minor product as a primary hydrogen atom is less reactive.

 
Chemistry Regents June 2006 - Question 61 PDF Print E-mail
chemjun06q61

Correct answer: Halogenation of an alkane

Why?:

Halogenation is an incorporation of a halogen atom into a molecule. Halogens are the elements under the group VIIA or group 17 of the periodic table. Examples are fluorine, chlorine, bromine and iodine.

Halogenation of an alkane is very hard to achieve because it requires harsh conditions such as very high temperature. The reaction mechanism involves free radical substitution. In this example, chlorine gas is broken down to form chlorine free radicals. Since chlorine is highly electronegative, the partially positive hydrogen on the alkane will be abstracted, making the alkane a radical as well. The process goes on similarly over and over again.

Answering this question:

Try to figure out first what kind of substances the reactants are. We have no problem with the Cl2. It is a gaseous halogen. C4H10 is an alkane as it is in accordance with the general formula for alkanes, CnH(2n+2). Since on the product side a chlorine atom has replaced one of the hydrogens in the alkane, it is a substitution reaction. And since the atom that has been incorporated into the alkane is a halogen, then it is a halogenation reaction.

So this reaction is halogenation of an alkane.

 
Chemistry Regents June 2006 - Question 55 PDF Print E-mail
chemjun06q55

Correct answer: alkanes

Why?:

A homologous series is a series of organic compounds with similar functional group, similar structure, and similar properties. Alkanes, alkenes, and alkynes are homologous series of hydrocarbons. They all have the carbon chain backbone. Alkanes have single bonds and no multiple bonds. Each carbon atom of an alkane is bonded to four other atoms such as hydrogen and another carbon. The general formula for straight-chain alkane is CnH(2n+2). Alkenes have at least one double bond between two carbon atoms. The general formula for alkenes is CnH2n. Alkynes on the other hand have at least one triple bond between two carbon atoms. The general formula for alkynes is CnH(2n-2).

Answering this question:

Count all the number of carbon atoms and hydrogen atoms. Decide which homologous series the formula of the given molecule follows. There are four carbon atoms and ten hydrogen atoms. The n value here is 4. If it follows the general formula for an alkane, the subscript of the hydrogen atom should be 2n+2=2(4)+2=10. Since the subscript of the hydrogen atom is 10, then it is an alkane. This is already enough to establish that the given molecule is an alkane. This is butane. You can draw its structure to confirm. If there is no double bond present when all the atoms are bonded together, then it is indeed an alkane.

So the given molecule is an alkane.

 
Chemistry Regents June 2006 - Question 45 PDF Print E-mail
chemjun06q45

Correct answer: (2) amide

Why?:

An aldehyde has the general formula (R=O)H. An amine has the general formula RNH2. An amide has R(C=O)NH2 and a ketone has R=O=R. R here is any alkyl or aryl group.

Answering this question:

Examine first the structure for its functional group. This structure can be classified on the basis of its functional group. It has a (C=O)NH2 group, which is called an amide.

So the correct answer is (2).
 
Chemistry Regents June 2006 - Question 43 PDF Print E-mail
chemjun06q43

Correct answer: (1) butanal

Why?:

Under the IUPAC naming system, molecules with a four-carbon chain backbone has the prefix "buta" in its name. Examples are butane, butanol, and butadiene. Those with a three-carbon chain backbone have "propa". Examples are propane, propanol, and propanal. Molecules containing an aldehyde functional group (CHO) have the "al" ending in their names. An example is propanal. Those containing the alcohol functional group (OH) have "ol" ending in their names. Examples are propanol and butanol.

Answering this question:

This molecule contains a four carbon chain backbone and an aldehyde functional group (CHO). So the name of this molecule has an ending "al" and a prefix "buta". Thus, the name is butanal.

So the correct answer is (1).

Last Updated on Wednesday, 23 December 2009 16:24
 
Chemistry Regents June 2006 - Question 22 PDF Print E-mail
chemjun06q22

Correct answer: (4) molecular formula

Why?:

Isomers are molecules having the same molecular formula but different structures. This difference could be about the placement or connections of atoms. Since isomers have the same molecular formula but different structures, they could also have similar, but not the same, properties such as density, melting and boiling points and reactivity.

Answering this question:

Given the formula of the two molecules being compared, we can easily notice that they have the same molecular formula by counting the number of atoms in each of them. Each of them contains 2 carbon atoms, 6 hydrogen atoms, and 1 oxygen atom. Even just from that, we can immediately conclude that (4) is the correct answer.

So (4) is the correct answer.

Last Updated on Wednesday, 23 December 2009 15:17
 
Chemistry Regents June 2006 - Question 20 PDF Print E-mail
chemjun06q20

Correct answer: (1) C3H4

Why?:

Propyne is an alkyne with three carbon atoms. Since it is an alkyne, one of the C-C bonds contains triple bonds. A carbon atom is capable of having four bonds. If carbon 1 in propyne is triply bonded to carbon 2, then carbon 1 is bonded to just 1 hydrogen atom. Since carbon 2 is already triply bonded to carbon 1 and it is bonded to carbon 3, then no hydrogen atom is bonded to carbon 2. Carbon 2 is singly bonded to carbon 3. Therefore carbon 3 is bonded to 3 more hydrogen atoms. To recap, there are 3 carbon atoms. For the hydrogen atoms, there is 1 on carbon 1, nothing on carbon 2, and 3 on carbon 3. So the formula is C3H4.

Answering this question:

Alkynes have the general formula CnH2n-2. Among the choices, only (1) and (2) contains 3 carbon atoms. Choices (3) and (4) contain 5 carbon atoms so neither of them is propyne. They can be eliminated.

In choices (1) and (2), get the n value. This is simply the subscript of the carbon atom. Then obtain the value for 2n-2 in each of the molecules and compare with those in the choices. In (1), n=3 and 2n-2 = 2(3)-2 = 4. Since the subscript of the hydrogen atom is 4, then it is the correct answer. In (2), n=3 and 2n-2 = 2(3)-2 = 4. Since the subscript of the hydrogen atom is 6, it is not the correct answer.

So the correct answer is (1).

Last Updated on Wednesday, 23 December 2009 15:14
 
Chemistry Regents June 2006 - Question 17 PDF Print E-mail
chemjun06q17

Correct answer: (2) CH3CH2CH2CH3

Why?:

Hydrocarbons are organic compounds composed solely of hydrogen and carbon atoms.

Answering this question:

As the name implies, hydrocarbons consist only of hydrogen and carbon atoms. The name hydrocarbon would seem to be a linkage of the words "hydrogen" and "carbon". From this, it will give us a hint that (2) is the correct answer. Choices (1), (3), and (4) can be eliminated because all of them contain oxygen atoms.

Therefore, (2) is the right answer.

Last Updated on Wednesday, 23 December 2009 15:06
 


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