Alkanes Nomenclature and Numbering
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The simplest organic compounds are hydrocarbons . Hydrocarbons contain only two elements , hydrogen and carbon . A saturated hydrocarbon or alkane is a hydrocarbon in which all of the carbon-carbon bonds are single bonds . Each carbon atom forms four bonds and each hydrogen forms a single bond to a carbon. The bonding around each carbon atom is tetrahedral, so all bond angles are 109.5 degrees. As a result, the carbon atoms in higher alkanes are arranged in zig-zag rather than linear patterns.
Straight-Chain Alkanes
The general formula for an alkane is C n H 2 n +2 where n is the number of carbon atoms in the molecule. There are two ways of writing a condensed structural formula . For example, butane may be written as CH 3 CH 2 CH 2 CH 3 or CH 3 (CH 2 ) 2 CH 3 .
Rules for Naming Alkanes
- The parent name of the molecule is determined by the number of carbons in the longest chain.
- In the case where two chains have the same number of carbons, the parent is the chain with the most substituents .
- The carbons in the chain are numbered starting from the end nearest the first substituent.
- In the case where there are substituents having the same number of carbons from both ends, numbering starts from the end nearest the next substituent.
- When more than one of a given substituent is present, a prefix is applied to indicate the number of substituents. Use di- for two, tri- for three, tetra- for four, etc. and use the number assigned to the carbon to indicate the position of each substituent.
Branched Alkanes
- Branched substituents are numbered starting from the carbon of the substituent attached to the parent chain. From this carbon, count the number of carbons in the longest chain of the substituent. The substituent is named as an alkyl group based on the number of carbons in this chain.
- The numbering of the substituent chain starts from the carbon attached to the parent chain.
- The entire name of the branched substituent is placed in parentheses, preceded by a number indicating which parent-chain carbon it joins.
- Substituents are listed in alphabetical order. To alphabetize, ignore numerical (di-, tri-, tetra-) prefixes (e.g., ethyl would come before dimethyl), but don't ignore don't ignore positional prefixes such as iso and tert (e.g., triethyl comes before tertbutyl).
Cyclic Alkanes
- The parent name is determined by the number of carbons in the largest ring (e.g., a cycloalkane such as cyclohexane).
- In the case where the ring is attached to a chain containing additional carbons, the ring is considered to be a substituent on the chain. A substituted ring that is a substituent on something else is named using the rules for branched alkanes.
- When two rings are attached to each other, the larger ring is the parent and the smaller is a cycloalkyl substituent.
- The carbons of the ring are numbered such that the substituents are given the lowest possible numbers.
Straight Chain Alkanes
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Chemistry Steps
Organic Chemistry
Alkanes and cycloalkanes.
In today’s post, we will talk about the IUPAC rules of nomenclature for naming alkanes and alkyl halides.
The first thing you need to do before learning the IUPAC rules for systematic nomenclature is making sure you know the names of the first ten alkanes:
Assuming you have already mastered those, let’s draw a structure and name it simply based on the molecular formula:
The compound has five carbons with no multiple bonds, therefore the formula is C 5 H 12, and based on the common names, we can see that it is pentane .
The complications and need for rules arise when the molecules get branched out. For example, what if we add a methyl (CH 3 ) group to pentane?
This group is considered a substituent ; an additional group that is on the “main part” of the molecule called the parent chain .
So, remember, we distinguish two units; the “main part” of the molecule, called the parent chain , and the additional group(s) known as substituents.
There are certain rules for determining the parent chain and the substituent(s) so let’s discuss them one-by-one and name this molecule (let’ name it molecule A ) in the course of doing that.
Substituents and Alkyl groups
The substituent can be a carbon fragment, and these are called alkyl groups , or any other functional group such as a halide, an OH, a nitro group, etc.
Alkyl groups are formed by removing one hydrogen from the corresponding alkane and are named based on this alkane by simply changing the ending from – ane to -yl .
Sec- , tert- and iso- prefixes
Just like the constitutional isomers , it is possible to have different alkyl groups with the same chemical formula.
For example, aside from the propyl group, there is also iso propyl. The difference is that in the isopropyl, a hydrogen, connected to a secondary carbon atom, is removed and it is this secondary carbon that is connected to the parent chain.
Likewise, the butyl group can also be primary, secondary, and tertiary. It will be very helpful to memorize all these groups and below is a general scheme to visualize how the names of these alkyl groups are derived:
You can also read this post about primary, secondary, and tertiary carbon atoms .
Identifying the Parent Chain
The parent chain is determined based on the longest continuous carbon chain that is present in the molecule. As an example, let’s consider molecule A mentioned earlier:
If we start numbering the carbon atoms from the methyl substituent, we can only have a continuous chain of four carbons. However, starting from any end allows making a five-carbon chain which is preferred since it makes a longer parent chain.
Therefore, the parent chain is pentane and the substituent is a methyl group.
If you run into a situation where there are two chains of equal length, then choose the one with the greater number of substituents :
When a ring is present, the parent chain is determined based on the number of carbons. If the ring has more carbons than the chain, then it is the parent chain:
Notice that the carbons in the ring belong to the ring only. I.e. you cannot count the carbon twice or include it in the carbon chain.
If the ring and the chain have an equal number of carbon atoms, the ring gets a priority and is considered as the parent chain.
Putting the parent chain and substituents together
When naming a compound, the alkyl groups are listed first followed by the parent chain. No sign or space needed to separate two words .
However, notice also that a number to specify the position of the alkyl groups is included in the final name. And this an important piece of information.
To illustrate this, let’s look at this example.
The following two compounds are both methylpentanes but they are clearly not identical:
And, in order to distinguish them, we need to specify the location of the methyl group. For this, the parent chain is numbered, and the rule here is to always do it such that the alkyl group gets the lowest possible number :
Starting from the left or the right side of the parent chain, we get two names and out of these, 2-methylpentane is better than 4-methylpentane. Therefore, 2-methylpentane is the correct IUPAC name of this compound.
Notice that for the second compound, it does not matter where we start the numbering, since it is a symmetric molecule, and either way the methyl group gets number 3:
A few additional details to point out when writing the name of a compound:
1) Numbers and words are separated by a dashed line
2) Words are not separated by any sing or a space
Parent chain with two substituents
Now, let’s add another methyl group next to the first one:
Again, you have two options for numbering the parent chain. One gives the 3,4 and the other one 2,3 locants for the two methyl groups and 2,3 clearly beats 3,4.
Therefore, the final name of our compound is going to be 2,3- di methylpentane.
Notice that numbers are separated by commas and because there are two methyl groups, we need to use the prefix “di” before the name of the alkyl groups.
In general, if two or more identical substituents are present, the corresponding prefixes are used to indicate their number:
Two – di Three – tri Four – tetra Five – penta Six – hexa Seven – hepta Eight – octa
Let’s also consider the other option of having two methyl groups on pentane:
Notice that, in this case, regardless of where we start the numbering, the first methyl gets locant 2, and the second one gets 4. Therefore, it is 2,4-dimethylpentane.
Now, the question comes – what if there is a third substituent and it does matter where to start numbering?
To answer this, let’s consider heptane with three methyl groups:
Starting from left or right makes no difference as far as having the location of the first substituent. Either way, it is 2. However, if you start from the left, you are getting 2,5,6-trimethylheptane, while starting from the right, gives 2,3,6-trimethylheptane.
And because 2,3,6 is better than 2,5,6, the correct name of this molecule is 2,3,6-trimethylheptane.
To summarize this observation, when there is a tie for the location of the first substituent, compare the second one, then the third till you find a tiebreak if there is one.
If numbering the alkyl groups does not break the tie, then the substituent with the alphabetical priority gets the lower locant:
For example, having a bromine and chlorine on both ends of hexane brings up the need of prioritizing the substituents based on their alphabetical order .
In this case, 1-bromo-6-chlorohexane beats 6-bromo-1-chlorohexane:
If none of the rules discussed above give a tiebreak, then it is a symmetrical molecule and it does not matter where you start numbering the parent chain – as long as you do find the correct parent chain.
Alphabetical order in IUPAC naming
So far, we have considered having identical alkyl groups. Now, suppose we need to name the following compound:
Step 1. Find the parent chain. The longest possible chain here consists of nine carbons, so the parent chain is nonane.
Step 2. Find the substituents. In this case, we have a methyl and an ethyl group.
Step 3. Number the parent chain giving the lowest possible numbers to the substituents:
Out of the two options, 2-methyl is better than 4-ethyl.
Step 4. Put the parent chain and substituents together by placing the substituents in alphabetical order!
This means that even though the methyl group is at position 2, the ethyl group with the locant 6 is still placed before it:
The alphabetical priority of prefixes
None of the prefixes such as di, tri, tetra, sec- , tert- are considered for alphabetical priority except the -iso.
For example:
Still looking forward to finding out why -iso is privileged…
Naming complex substituents
Sometimes, we run out of the common names for the substituents such as sec -butyl, tert -butyl, iso -butyl but we still need to name a substituent that is larger than usual.
For example, in the following molecule, it is easy to spot the ethyl group but a naming the second substituents needs to follow certain rules.
The good news is that these rules are no different than what we use when naming a compound. Essentially, you need to look at the complex substituent as a separate molecule and find its “parent chain” and the alkyl groups on it.
To do this, start numbering from the carbon directly connected to the actual parent chain of the molecule and list the alkyl groups alphabetically:
Notice that at the end, the quasi parent chain gets the -yl suffix since it is still a substituent and the actual parent chain is placed at the end.
This systematic approach for naming alkyl groups can also be applied for the ones with common names and you will likely need to know both options.
In the following practice problems, we will go over naming alkanes using the IUPAC nomenclature rules which include finding the parent chain, numbering it to have the substituents in the correct positions, and finally putting all of this together to name the compound.
The next exercise will teach you to draw the structure based on the IUPAC name.
Identify and name the parent in each of the following compounds:
Provide a systematic name for each of the following compounds:
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Draw the bond-line structure for each of the following molecues:
a) 4-ethyl-5-isopropyl-3-methyloctane
b) 1-ethyl-4-methyl-2-propylcyclopentane
c) 5-( sec -butyl)-8-isopropyl-3,4,9-trimethylundecane
d) 6-( sec -butyl)-7-ethyl-3,4-dimethyldecane
e) 3-ethyl-4,5-dimethylheptane
f) 4-ethyl-3-isopropyl-1,1-dimethylcyclohexane
g) 6,6-diethyl-3,5,5-trimethylnonane
h) 4-( sec -butyl)-3,3,5,5-tetramethylheptane
Check Also:
- Naming Bicyclic Compounds
- Naming Bicyclic Compounds-Practice Problems
- How to Name a Compound with Multiple Functional Groups
- Primary Secondary and Tertiary Carbon Atoms in Organic Chemistry
- Constitutional or Structural Isomers with Practice Problems
- Degrees of Unsaturation or Index of Hydrogen Deficiency
- Newman Projections with Practice Problems
- Gauche Conformation, Steric, Torsional Strain Energy Practice Problems
- Drawing the Chair Conformation of Cyclohexane
- Ring Flip: Drawing Both Chair Conformations with Practice Problems
- 1,3-Diaxial Interactions and A value for Cyclohexanes
- Ring-Flip: Comparing the Stability of Chair Conformations with Practice Problems
- Cis and Trans Decalin
3 thoughts on “Naming Alkanes with Practice Problems”
Can you please tell me the proper name of this compound: (CH3 )3 C-CH2-CH(Cl)-(CH2)3-C(CH3)2-CH(NH2)-C(C2H5)(CH3)2
You need to first convert this into a bond-line structure to find the parent chain and substituents.
9-amine-4-chloro-2,2,8,8,10,10-hexamethyl n-dodecane
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Select the correct structure for the IUPAC names given below:
Part 1. 4-methylheptane Choose a Compound Compound A Compound B Compound C Compound D Compound E Compound F Compound G Compound H Compound I Compound J Compound K Compound L Part 2. 2,4-dimethylhexane Choose a Compound Compound A Compound B Compound C Compound D Compound E Compound F Compound G Compound H Compound I Compound J Compound K Compound L Part 3. 2,3-dimethylpentane Choose a Compound Compound A Compound B Compound C Compound D Compound E Compound F Compound G Compound H Compound I Compound J Compound K Compound L Part 4. 2,2-dimethylhexane Choose a Compound Compound A Compound B Compound C Compound D Compound E Compound F Compound G Compound H Compound I Compound J Compound K Compound L Part 5. 2-ethyl-1,1,3,3-tetramethylcyclopentane Choose a Compound Compound A Compound B Compound C Compound D Compound E Compound F Compound G Compound H Compound I Compound J Compound K Compound L Part 6. 3-ethyl-2-methylpentane Choose a Compound Compound A Compound B Compound C Compound D Compound E Compound F Compound G Compound H Compound I Compound J Compound K Compound L
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2.5: Practice - Alkane Nomenclature
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I. The parent compound must have the longest chain of carbon atoms
Learn: methane (1 carbon atom), ethane (2 carbon atoms), propane (3 carbon atoms), butane (4 carbon atoms), pentane (5 carbon atoms), hexane (6 carbon atoms), heptane (7 carbon atoms), octane (8 carbon atoms), nonane (9 carbon atoms) and decane (10 carbon atoms).
Try to name the following compounds:
- CH 3 -CH 2 -CH 2 -CH 3
- CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3
- CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3
Try to draw structures for the following compounds:
II. The parent chain is numbered to give substituents the lowest possible numbers
Substituent names are methyl, ethyl, propyl, butyl, etc. The number showing the point of attachment to the parent chain precedes the substituent name. If you have more than one substituent with the same name, a number of attachment must be given for each substituent and the number of the substituents must be designated with di-, tri-, etc.
Try to name the following compounds...
Try to draw structures for the following compounds...
- 2,2-dimethylpropane
- 3-methylheptane
- 4,5-diethylnonane
III. Substituents are named in alphabetical order
Remember that the numbering of the parent chain must give all substituents the lowest possible numbers regardless of their names.
Try to name the following compound...
IV. A large substituent is numbered to give its point of attachment
The point of attachment is number one and any other smaller groups are named as substituent groups on the larger group. This numbering is independent of the numbering of the parent chain. Try to name the following compounds...
- 4-(1-methylethyl)heptane
- 5-(1,1-dimethylethyl)nonane
V. Ring compounds are designated with a cyclo- prefix and are numbered to give multiple substituents the lowest possible numbers
A single substituent does not need to be numbered. A ring can also be named as a substituent. Try to name the following compounds:
- ethylcyclobutane
- 1-ethyl-4-methylcyclohexane
- cyclobutylcyclooctane
VI. Common Names
- isopropyl = 1-methylethyl
- isobutyl = 2-methylpropyl
- sec -butyl = 1-methylpropyl
- tert -butyl = 1,1-dimethylethyl
- neo -pentyl = 2,2-dimethylpropyl
- iso -pentyl = 3-methylbutyl
Try to name the following compounds using common names...
- 4-isopropyloctane
- isopentylcyclohexane
- CH 3 -CH 2 -CH 3
- CH 3 -CH 2 -CH 2 -CH2-CH 3
- CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3
- 2-methylbutane
- 3-ethylhexane
- 3-methyloctane (Remember that you must number the parent chain to give the substituent the lowest possible number.)
- 4-ethyl-3-methylheptane
4-(1-methylethyl)octane
5-(1-methylpropyl)decane
1-ethyl-2-methylcyclohexane
methylcyclopentane (You do not need a number since there is only one substituent and it is assumed to be on the first carbon regardless of how the ring compound is drawn.)
cyclopropylcyclopentane
neo -pentylcycloheptane (1-(2,2-dimethyl-1-propyl)cycloheptane is the proper name.)
isopropylcyclohexane
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Richard Banks ( Boise State University )
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Organic Chemistry Nomenclature Practice – Naming Alkanes
An iupac name for alkane spaghetti, organic chemistry naming practice with alkane spaghetti.
7) Put it together
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