Special Sub-Topic: Tricky Hydrocarbon Names and Numbers
|Which of the following is the only isomer of butane, and is also referred to as isobutane?|
2-methylpropane. Alkane isomers are formed when a methyl particle (CH3) attaches itself to the middle of a regular alkane. The name of the isomer is formed in the following way:
position + number of alkyl particles + type of alkyl particle (s) + name of longest chain.
In this case, there is only one methyl particle so there is no prefix to indicate the number, as well as only one position (2), since there is only one place where it could go, and the longest chain has three carbon atoms.
Therefore, the alkane with 4 carbon atoms, which is an exact isomer of butane, is 2-methylpropane.
Isobutane is now being widely used as a gas in refrigerating systems as a substitute to CFCs.
|Which of the following is not a possible isomer of hexane?|
methylbutane. To figure out the answer one can follow the following rule:
number of alkyl particles (no prefix = 1, di = 2, tri = 3, etc..) multiplied by the type of alkyl particles (methyl = 1, ethyl = 2, propyl = 3, etc..) added to longest carbon chain (propane = 3, butane =4, pentane = 5)
Hexane has 6 carbon atoms, and following the above formula:
- 2,3-dimethylbutane = 2 X 1 + 4 = 6
- 2-methylpentane = 1 + 5 = 6
- 3-methylpentane = 1 X 1 + 5 = 6
Therefore, these are all isomers of hexane.
However, methylpropane with only 5 carbon atoms, is not an isomer of hexane, and is instead an isomer of pentane.
|How many carbon atoms does 3,4-dimethyloctane have?|
10. Using the above formula, the carbon number of 3,4 dimethyloctane can be obtained as such:
Di (2) X methyl (1) + octane (8), which leaves us with 10 carbon atoms.
Dimethyloctane is one of many isomers of decane, and they both have the molecular formula of C10H22.
|Which of the following hydrocarbons has 9 carbon atoms?|
3,4-dimethylheptane. Following the previous formula, we can conclude that:
2-3-dimethylhexane has 8 carbon atoms: (2 X 1 + 6 = 8)
3-methyldecane has 11 carbon atoms: (1 X 1 + 10= 11)
3-4-dimethylheptane has 9 carbon atoms:(2 X 1 + 7 = 9)
Therefore, 2,3-dimethylhexane is an isomer of octane, 3-methyldecane is an isomer of undecane, and 3,4-dimethylheptane is an isomer of nonane.
|How many carbon atoms would be in 3,5-diethyl-undecane? |
15. Following the same formulas as before, the number of carbon atoms in this isomer can be calculated:
Number of alkyl particles = 2 (use of the word "di")
Type of alkyl particle = 2 (use of the word ethyl)
Longest carbon chain = 11 (use of the word undecane)
Therefore, the total number of carbon atoms is 15. (2 X 2 + 11 = 15).
Of course, this would make 3,5-diethyl-undecane an isomer of pentadecane, which is the straight-chained alkane with a molecular formula of C15H32.
|Which of the following straight-chained alkanes has the most isomers? |
hentriacontane. All of these are straight-chained alkanes, and the more the number of carbon atoms, the greater the number of isomers that the alkane has. Tricosane, which has 23 carbon atoms, and a molecular formula of C23H48, has a little under 6 million isomers. Docosane, which has 22 carbon atoms, and a molecular formula of C22H46, has a little over 2 million isomers. Triacontane, which has 30 carbon atoms, and a molecular formula of C30H62 has just over 4 billion isomers. The winner, hentriacontane with 31 carbon atoms (C31H64) has a little over 10.5 billion isomers.
A fun yet scary fact: Huge alkanes such has Nonahectane (C109H220) and Decahectane (C110H222) have over 10,000,000,000,000,000,000,000,000,000,000,000,000,000 isomers!
(Before sending me correction notes teaching me about scientific notation, please note that all those zeroes are for effect:) )
|Which of the following hydrocarbons has more than 20 carbon atoms? |
2,3,4,6-tetramethylnonadecane. The calculations are as following:
3,4-diethylpentadecane has 19 carbon atoms: (2 X 2 + 15 = 19), so it cannot be the answer. It is an isomer of nonadecane. (C19H40)
4-methylheptadecane has 18 carbon atoms: (1 X 1 + 17 = 18), so it cannot be the answer. It is an isomer of octadecane. (C18H38)
5-ethyloctadecane has 20 carbon atoms: (1 X 2 + 18 = 20), so it cannot be the answer. It is an isomer of Eicosane. (C20H42)
However, 2,3,4,6-tetramethylnonadecane has 23 carbon atoms: (4 X 1 + 19 = 23), and it is the only compound with more than 20 carbon atoms, and therefore the correct answer. It is an isomer of tricosane (C23H48)
|2,3-dipropylnonadecane has fewer than 23 carbon atoms. |
F. Absolutely not!
There are two propyl groups in this compound, each of which has 3 carbon atoms, for a total of 6 carbon atoms attached to the longest chain, which we have established is nonadecane. Therefore, the total number of carbon atoms in this compound is 25, making it one of over 36 million isomers of pentacosane (C25H52).
2 (Di) X 3 (Propyl) + 19 (nonadecane) = 25.
|Which of these isomers cannot exist? |
6,7-dimethylpropane. All the above compounds can exist in nature, as 2,3-dimethylpentane is an isomer of heptane, 2-ethylhexane is an isomer of octane, and 2-methylbutane is an isomer of pentane. However, 6,7-dimethyl propane is an impossible compound, because the longest chain is 3, so it is impossible to have two methyl particles attached to carbons 6 and 7 of the longest chain, especially when the total number of carbons in the compound is 5. In addition, two methyl particles cannot attach themselves to a three-chain hydrocarbon, since that would automatically make the longest chain 4, thus creating methylbutane.
|Which of the following is an isomer of nonadecane?(C19H40)|
all of these (3-methyloctadecane, 2,4-dimethylheptadecane, 5-ethyl-3-propyltetradecane). 3-methyloctadecane has 19 carbon atoms, since the longest chain has 18, as well as one more attached to it in the form of a methyl particle.
Similarly, 2,4-dimethylheptadecane, has 19 carbon atoms, since the longest chain has 17, as well as 2 more attached to it in the form of two seperate methyl groups.
To end with a tricky one, 5-ethyl-3-propyltetradecane also has 19 carbon atoms, following the formula:
1 (no prefix) X 3 (propyl) + 1 (no prefix) X 2 (ethyl) + 14 (tetradecane) = 19.
Therefore, all these have exactly 19 carbon atoms and 40 hydrogen atoms, which makes them isomers of nonadecane.
Hope you enjoyed the quiz, and that you learned a thing or two about organic chemistry.
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