IUPAC Naming Branched-Chain Alkanes Chemistry Tutorial

Key Concepts

• Nomenclature is a system of rules by which the names of compounds are formed.
• The International Union of Pure and Applied Chemistry (IUPAC) is developing the rules for naming compounds.⁽¹⁾
• For nomenclature purposes, all compounds containing carbon are said to be organic compounds.
• Alkanes are organic molecules, molecules containing carbon.
• Alkanes are classed as hydrocarbons, molecules containing ONLY carbon and hydrogen atoms.

Naming Branched-Chain Alkanes:

• Name of a branched-chain alkane is made up of two parts:

  alkyl	alkane
  branch name	stem name
  prefix	suffix

• Name the longest alkane carbon chain (stem) first.
• Identify the branches (side chains).⁽²⁾
• Name each branch (side chain).
• Number the carbon atoms making up the stem so that the branches have the lowest possible numbers.
• Separate the number of the stem carbon atom from the name of the branch by a hyphen (-).

  If two or more identical branches are present:

  (i) Use the prefixes di, tri, tetra, etc before the branch name.
  (ii) Indicate the location of the stem carbon atoms to which each branch is attached by writing the numbers separated by a comma (,) in ascending order.
  (iii) Separate the final number from the prefix of the branch name by a hyphen (-).

  If two or more different branches (side chains) are present:

  (i) Identify and name each branch (side chain).
  (ii) Indicate the location of the stem carbon atoms to which each branch is attached by writing the number then a hyphen and then the name of the branch.
  (ii) Write the names of the branches in alphabetical order (ignoring any di, tri, tetra, etc prefix) separating numbers from letters by a hyphen.

• Branch name(s) added as a prefix to the name of the stem.

Drawing the structure⁽³⁾ of branched-chain alkanes:

• Divide the name into the name of the branches and the name of the stem.
• Draw the straight chain alkane stem.
• Number the carbon atoms making up the stem's chain.
• Break the rest of the name up into individual branch names.
• Determine the number of identical branches (if any).
• Determine the location of the carbon atom on the stem to which each branch (side chain) is attached.
• Note the locations of the branches on the numbered stem chain you have already drawn.
• Draw the required number of carbon atoms for each branch at the specified location along the stem's carbon chain.
• Complete the structure by adding a hydrogen atom at the end of any vacant covalent bond.

Molecular formula of branched-chain alkanes:

• Draw the structure of the branched-chain alkane.
• Write a skeletal molecular formula using the symbols for carbon and hydrogen:

  C   H

• Count the number of carbon atoms in total in the structure.

  total number of carbon atoms = n

• Write the number of carbon atoms as a subscript number to the right of the symbol for carbon

  C_nH

• Count the number of hydrogen atoms in total in the structure.

  total number of hydrogen atoms = y

• Write the number of hydrogen atoms as a subscript number to the right of the symbol for carbon

  C_nH_y

• The general molecular formula for a branched-chain alkane is C_nH_2n+2

  where n = number of carbon atoms in the carbon chain

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Bonding and Structure of Branched Chain Alkanes

Branched-chain alkanes are hydrocarbons, that is, molecules containing ONLY carbon atoms and hydrogen atoms.

• Each carbon atom in the alkane molecule has 4 valence electrons.

  	.
  .	C	.
  	.

• Each hydrogen atom in the alkane molecule has 1 valence electron.

  .

  H

• Each carbon atom in the alkane molecule makes 4 covalent bonds by sharing a valence electron with another atom to make an octet of electrons (4 electron pairs).

  Lewis Structure (electron dot diagram)		Valence Structure
  .   .   . . C . .   .   .		|   - C -   |

• Each carbon atom can covalently bond to hydrogen atoms and to other carbon atoms:

  	C-C		C-H
  Lewis Structure	. . C . .. . C . .		. . C . .. H
  Valence Structure	. . C . - . C . .		. . C . - H

• If an alkane molecule contains 1, 2, or 3 carbon atoms, ONLY straight chain alkanes are possible⁽⁴⁾.

  No. carbon atoms	Lewis Structure	Valence Structure	Name
  1	H     H .. .. C .. .. H     H	H   H | -C- | H   H	methane
  2	H   H     H. . .. C .. .. .. C .. . .H     H   H	H   H     H - | C | - | C | - H     H   H	ethane
  3	H   H   H     H. . .. C .. .. .. C .. . .. C .. . .H     H   H   H	H   H   H     H - | C | - | C | - | C | - H     H   H   H	propane

• If an alkane molecule contains 4 or more carbon atoms, it is possible to draw structures that include branches, or side-chains.

  Branches or side-chains occur when a non-terminal carbon atom (a carbon atom that is NOT at the ends of a chain) covalently bonds to 3 or 4 carbon atoms.

  A fourth carbon atom could be added to propane, C₃H₈ (shown above), either to make a 4 carbon straight chain alkane:

  Lewis Structure		Valence Structure
  H   H   H   H     H. . .. C .. .. .. C .. . .. C .. . .. C .. . .H     H   H   H   H		H   H   H   H     H - | C | - | C | - | C | - | C | - H     H   H   H   H

  Or, a fourth carbon atom could be added to the second carbon atom in the propane chain:

  Lewis Structure		Valence Structure
  H   H   H     H. . .. C .. .. .. C .. . .. C .. . .H     H   H..C..H   H             .. H		H   H   H     H - | C | - | C | - | C | - H     H   H-C-H   H             | H

  This is known as a branched-chain alkane.
  There is a "branch", or a side chain, of carbon atoms coming off the longest straight carbon chain.

  A branched-chain alkane is made up of a "stem" of carbon atoms in a straight chain, and, a "branch" of 1 or more carbon atoms coming off the stem.

  branch containing 1 carbon atom

  C |

  stem containing 6 carbon atoms

  -

  C

  -

  C

  -

  C

  -

  C

  -

  C

  -

  C

  -

  branch containing 2 carbon atoms

  | C | C

  Remember, the branches or side chains are shorter in length (that is, have fewer carbon atoms in the chain) than the stem (longest carbon chain).

Steps for Naming Branched-Chain Alkanes

Step 1. Name the stem, the longest carbon chain first.
The stem of the branched-chain alkane is named in the same way as you learned to name straight chain alkanes.

Step 2. Identify the branches that occur along the length of the stem (the longest carbon chain).

Step 3. Name each branch (or side chain).

The branches (or side chains) are named depending on the number of carbon atoms in the chain.

The general name of a branch or side chain is alkyl

The name of a branch, or side chain, is made up of 2 parts:

    (i) a prefix (alk) that tells us how many carbon atoms are in the chain

    (ii) the suffix "yl"

Step 4. Number the carbon atoms in the stem (longest carbon chain) so that branches (or side chains) are given the lowest possible numbers.

Step 5. Indicate where the branch (side chain) occurs along the stem:

    Note the number of the carbon atom on the "stem" to which the branch is attached.

    Write this number, then a hyphen (-), before the name of the branch

Step 6 (a). If only one branch (or side chain) is present, complete the name by adding the name of the branch as a prefix to the name of the stem as a suffix

Step 6 (b). If two or more branches (side chains) have the same number of carbon atoms:

• Use the prefixes di, tri, tetra etc to denote the number of identical branches (chain sides):

  number of identical branches	prefix
  2	di
  3	tri
  4	tetra

  Example:						H |				Two identical branches each containing 1 carbon atom.
  		H |		H |		H-C-H |		H |		Two methyl branches off a butane stem
  	H-	C	-	C	-	C	-	C	-H	two = di
  		| H		| H-C-H		| H		| H
  				| H

• Write this prefix before the name of the branch (side chain):

  Example:     dimethyl

• Write the number of the stem carbon atom to which each branch is attached, separated by a comma (,) in ascending order (lowest number first):

  Example:						H |				methyl branches attached to carbon atoms 2 and 3 of the stem
  		H |		H |		H-C-H |		H |		2,3
  	H-	C1	-	C2	-	C3	-	C4	-H
  		| H		| H-C-H		| H		| H
  				| H

• Write a hyphen (-) between the last number and the prefix denoting the number of identical branches (side chains):

  Example:     2,3-dimethyl

• Write the complete name by writing all of the above as a prefix to the stem name:

  Example:	2,3-dimethyl	butane
  	branches	stem

Step 6 (c) If there are two or more different branches (side chains) present:

• Identify and name each branch:

  Example:

  H |

  Two different branches.

  H |

  H |

  H |

  H-C-H |

  H |

  One branch has only 1 carbon atom in the chain.

  H-

  C

  -

  C

  -

  C

  -

  C

  -

  C

  -H

  1 carbon branch is methyl

  | H

  | H

  | H-C-H

  | H

  | H

  The other branch has two carbon atoms in the chain.

  | H-C-H

  2 carbon branch is ethyl

  | H

• Number the carbon atoms of the stem so that the branches are given the lowest possible combination of numbers.

  H |         H |   H |   H |   H-C-H |   H |   H- C5 - C4 - C3 - C2 - C1 -H   | H   | H   | H-C-H   | H   | H             | H-C-H                     | H	NOT	H |         H |   H |   H |   H-C-H |   H |   H- C1 - C2 - C3 - C4 - C5 -H   | H   | H   | H-C-H   | H   | H             | H-C-H                     | H
  combination of 2 and 3	is less than	combination of 3 and 4

• Add the number of each stem carbon atom to the name of the branch separated by a hyphen (-):

  Example:                            
  2-methyl

  3-ethyl

• Assemble the prefix of the molecule's name by placing the names of the branches in alphabetical order (ignoring any di, tri, tetra etc prefixes) separated by a hyphen (-):

  3-ethyl-2-methyl       NOT       2-methyl-3-ethyl

  because e (ethyl) comes before m (methyl) in the alphabet

• Add this complete prefix to the name of the stem as a suffix.

  3-ethyl-2-methyl	pentane
  branches	stem

Worked Example of Naming a Branched Chain Alkane

Name the molecule shown below:

Step 1. Name the longest alkane carbon chain (stem) first.

• 6 carbon atoms in the longest carbon chain.
          6 carbon atoms prefix = hex
• Only single bonds between carbon atoms in the chain
          suffix = ane
• Name of longest alkane carbon chain, stem, is hexane

Step 2. Identify the branches (side chains).

Step 3. Name each branch (side chain).

Step 4. Number the carbon atoms making up the stem so that the branches have the lowest possible numbers.

Step 5. Separate the number of the stem carbon atom from the name of the branch by a hyphen (-).

Step 6 (a) Branch name(s) added as a prefix to the name of the stem.

More than 1 branch is present, go to step 6 (b)

Step 6 (b) If two or more identical branches are present:

(i) Use the prefixes di, tri, tetra, etc before the branch name.

Two methyl branches
dimethyl

(ii) Indicate the location of the stem carbon atoms to which each branch is attached by writing the numbers in ascending order separated by a comma (,).

3,4

(iii) Separate the final number from the prefix of the branch name by a hyphen (-).

3,4-dimethyl

Step 6 (c) If two or more different branches (side chains) are present:

(i) Identify and name each branch (side chain).

1 × ethyl
2 × methyl named above as 3,4-dimethyl

(ii) Indicate the location of the stem carbon atoms to which each branch is attached by writing the number then a hyphen and then the name of the branch.

3-ethyl
3,4-dimethyl

(ii) Write the names of the branches in alphabetical order (ignoring any di, tri, tetra, etc prefix) separating numbers from letters by a hyphen.

3-ethyl-3,4-dimethyl

Write the complete name by adding the prefix (name of all the branches) to the suffix (name of the stem alkane)

3-ethyl-3,4-dimethylhexane

Steps for Drawing the Structure of Branched-Chain Alkanes

Step 1: Divide the name of the branched-chain alkane into two parts:

Step 2: Draw the straight chain alkane stem

Example: 2,2-dimethylpropane

• stem = propane
• prefix = number of carbon atoms in chain = 3 (prop)
• suffix = type of bonds between carbon atoms in chain = single bonds (ane)

Note that each carbon atom can make 4 covalent bonds and that we are drawing in all 4 bonds, even the ones that are not joining two carbon atoms together in the chain.

Step 3: Number the carbon atoms in the stem

Step 4: Break the the branch name up into individual branch (side chain) names:

2,2-dimethyl

Only one type of branch (side chain): methyl

Methyl branches contain only 1 carbon atom.

Step 5: Determine the number of identical branches (side chains):

2,2-dimethyl

There are two methyl groups: di

Step 6: Determine the location of the carbon atom on the stem to which each branch (side chain) is attached:

2,2-dimethyl

Both methyl branches are attached to carbon number 2 along the stem: 2,2-

Step 7: Identify the location of each branch along the stem:

Both methyl branches (side chains) will be attached to C²

Step 8: Draw in the required number of carbon atoms for each branch at the specified location along the stem's carbon chain:

Notice that we are still drawing in all 4 covalent bonds for each carbon atom, even when the bond is not between two carbon atoms.

Step 9: Complete the structure by placing a hydrogen atom (H) at the end of any vacant covalent bond:

Steps for Writing the Molecular Formula of Branched-Chain Alkanes

Step 1: Draw the structure of the branched alkane molecule.

Step 2: Write a skeleton molecular formula using the symbols for carbon (C) and hydrogen (H)

Step 3: Count the total number of carbon atoms in the branched-chain alkane molecule.

Step 4: Write the number of of carbon atoms into the skeleton molecular formula as a subscript number to the right of the symbol for carbon (C).

Step 5: Count the total number of hydrogen atoms in the branched-chain alkane molecule.

Step 6: Write the number of of hydrogen atoms into the skeleton molecular formula as a subscript number to the right of the symbol for hydrogen (H).

Step 7: Check that your completed molecular formula makes sense (C_nH_2n+2)

Example of Writing the Molecular Formula of a Branched Chain Alkane

Write the molecular formula for 2-methylpropane.

Step 1: Draw the structure of the branched alkane molecule.

Step 2: Write a skeleton molecular formula using the symbols for carbon (C) and hydrogen (H)

C H

Step 3: Count the total number of carbon atoms in the branched-chain alkane molecule.

There are a total of 4 carbon atoms in the molecule.

Step 4: Write the number of of carbon atoms into the skeleton molecular formula as a subscript number to the right of the symbol for carbon (C).

C₄H

Step 5: Count the total number of hydrogen atoms in the branched-chain alkane molecule.

There are a total of 10 hydrogen atoms in the molecule.

Step 6: Write the number of hydrogen atoms into the skeleton molecular formula as a subscript number to the right of the symbol for hydrogen (H).

C₄H₁₀

Step 7: Check that your completed molecular formula makes sense

General formula for an alkane: C_nH_2n+2

when n = 4

2n + 2 = (2 × 4) + 2 = 8 + 2 = 10

So, C₄H₁₀ is the molecular formula for the alkane

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