Learning Arc 01 · Read and classify carbon-halogen structures

Read the carbon-halogen bond

If the carbon-halogen bond is read incorrectly, the name, classification and predicted reaction can all be wrong.

Foundation for this Concept15 symbols · 5 recovery lessons · 3 entry checksOpen

Start here: words and symbols used in this Concept

Open only the recovery material you need before the Mystery.

Read these words and symbols first

hydrocarbon
A compound containing only carbon and hydrogen atoms.
CH3-Br is not a hydrocarbon because it also contains Br.
carbon skeleton
The connected pattern of carbon atoms in a molecule.
Hydrogen and halogen atoms are not part of the carbon skeleton.
open carbon chain
A connected carbon pattern whose ends do not join to make a closed loop.
A one-carbon skeleton also has no closed loop.
carbon ring
A connected carbon pattern containing a closed path back to its starting carbon.
A carbon ring is not an open carbon chain.
alkane
An open-chain hydrocarbon containing only single bonds.
A closed carbon ring is not an alkane in this beginner naming sequence.
haloalkane
A compound obtained from an alkane when one or more hydrogen positions are occupied by F, Cl, Br or I.
CH3-Br is a haloalkane.
CH4 is an alkane, not a haloalkane.
Carbon-ring cases are classified separately in Concept 1.5.
placeholder
A letter that temporarily stands for a group identified in each concrete example.
The placeholder is not an extra atom in the molecule.
halogen
One atom of F, Cl, Br or I in this Course.
A halogen is not part of the carbon skeleton.
attachment point
The place where a group forms a bond to the rest of a molecule.
The terminal dash marks an open bond.
The dash is not a minus sign.
The dash is not an extra atom.
alkyl group
The group obtained when one hydrogen position of an alkane becomes an attachment point.
Methane gives the methyl group CH3-.
CH3- is not a complete isolated molecule in this notation.
The naming arrow is not a chemical reaction.
R
Read R as ‘ar.’
R is a placeholder for the stated carbon-containing group.
In CH3-Br, R represents CH3-.
R is not an element.
R means only the carbon-and-hydrogen group identified for the current structure.
X
Read X as ‘ex.’
X is a placeholder for one halogen atom.
In CH3-Br, X represents Br.
X is not a new element.
Identify the actual halogen in each structure.
R-X
Read R-X as ‘R single-bond X.’
The dash joins R's attachment carbon directly to X.
In CH3-Br, R = CH3-.
In CH3-Br, X = Br.
State the actual R group before solving.
State the actual halogen before solving.
functional group
An atom, group or bond pattern used to predict a characteristic reaction family.
In CH3-Br, the C-Br bond is the functional-group site.
CH4 has no C-X bond.
Therefore CH4 does not contain the haloalkane functional group.
The same C-X pattern must still be classified by its actual carbon setting.
C1, C2, C3
Temporary labels that let us point to different carbon atoms in one drawing.
These labels are not automatically IUPAC position numbers and the digits are not atom counts.

Foundation 1

Temporary labels for this problem

C1 and C2 are short solving labels for the two carbon atoms in this drawing.

Do

  • Point to C1.
  • Point to C2.
  • Do not treat the digits as atom counts or final name numbers.
Structure of a two-carbon molecule. C1 has three hydrogens and a bond to C2. C2 has two hydrogens, a bond to C1 and a bond to Br.

Foundation 2

Build the chapter language

  • A hydrocarbon contains only carbon and hydrogen.
  • Its connected carbon pattern is the carbon skeleton.
  • An open chain has no closed carbon loop.
  • An open-chain hydrocarbon with only single bonds is an alkane.

Do

  • Find the one-carbon skeleton in methane.
  • Confirm that every methane bond is a single bond.
A carbon atom is at the centre. Four separate single bonds join it to four hydrogen atoms.

Foundation 3

Read an alkyl attachment group

  • Use one H position of an alkane as an attachment point.
  • The remaining group receives an alkyl name.
  • Methane, CH4, gives the methyl group CH3-.
  • Ethane, CH3-CH3, gives the ethyl group CH3-CH2-.
  • These are naming relationships, not reactions.

Do

  • Read CH3- as methyl attachment group.
  • Read CH3-CH2- as ethyl attachment group.
  • Read the final dash as an open bond.
  • Do not read the dash as a minus sign.

Green focus — The highlighted single bond is the C-Br functional-group bond represented by R-X.

Expanded bromomethane structure. Carbon has three hydrogen bonds and one highlighted bond to bromine.

Foundation 4

Decode R-X once

  • R and X are placeholders.
  • In CH3-Br, R represents CH3-.
  • In CH3-CH2-Br, R represents the ethyl group CH3-CH2-.
  • In both examples, X represents Br.
  • The final dash of R joins the single C-Br bond.

Do

  • Point to the group represented by R.
  • Point to the atom represented by X.
  • Point to their attachment bond.

Green focus — The highlighted single bond is the C-Br functional-group bond represented by R-X.

Expanded bromomethane structure. Carbon has three hydrogen bonds and one highlighted bond to bromine.

Foundation 5

Locate the haloalkane functional group

  • A haloalkane comes from an alkane when one or more H positions are occupied by halogen atoms.
  • CH3-Br is a haloalkane.
  • Its C-Br bond is the functional-group site.
  • CH4 has no C-X bond.
  • Ring cases are taught separately in Concept 1.5.

Do

  • Point to the C-Br bond in CH3-Br.
  • Check that no C-X bond exists in CH4.
  • Leave ring cases for Concept 1.5.

Green focus — The highlighted single bond is the C-Br functional-group bond represented by R-X.

Expanded bromomethane structure. Carbon has three hydrogen bonds and one highlighted bond to bromine.
A carbon atom is at the centre. Four separate single bonds join it to four hydrogen atoms.

Entry check 1

  • For CH3-Br:
  • What exact group does R represent?
  • What atom does X represent?
  • What does the dash represent?
Show the recovery
  • R represents CH3-.
  • X represents Br.
  • The dash represents one single C-Br bond.
  • Return to the expanded structure.
  • Point to the carbon group.
  • Point to the Br atom.
  • Point to their shared bond.

Green focus — The highlighted single bond is the C-Br functional-group bond represented by R-X.

Expanded bromomethane structure. Carbon has three hydrogen bonds and one highlighted bond to bromine.

Entry check 2

In H-Cl, which two printed atom symbols are joined by the dash?

Show the recovery

H and Cl.

  • H and Cl are element symbols.
  • Point to H.
  • Trace the dash once.
  • Stop at Cl.
  • The dash is one covalent bond.

Entry check 3

  • The expanded methane model shows four C-H bond lines.
  • What ordinary valency does carbon show?
  • How many bond lines touch Cl in H-Cl?
Show the recovery
  • Carbon shows valency four.
  • Cl shows valency one in these neutral structures.
  • Count the bond lines touching carbon in methane.
  • Four bond lines touch carbon.
  • Count the bond lines touching Cl in H-Cl.
  • One bond line touches Cl.
A carbon atom is at the centre. Four separate single bonds join it to four hydrogen atoms.
1 · ObjectiveOpen

What you will understand

Identify exactly which carbon and halogen are joined by a printed bond.

Problems this Concept unlocks

  • identify the carbon attached to one of the chapter halogens
  • separate atom counts from bond connections
  • find the carbon-halogen bond used by later reaction problems
2 · MysteryOpen

The problem to explain

Read these words and symbols first

C1, C2, C3
Temporary labels that let us point to different carbon atoms in one drawing.
These labels are not automatically IUPAC position numbers and the digits are not atom counts.
  • This drawing shows every atom and bond.
  • C1 and C2 are temporary labels for the two carbon atoms.
Structure of a two-carbon molecule. C1 has three hydrogens and a bond to C2. C2 has two hydrogens, a bond to C1 and a bond to Br.
  • The molecule contains two carbon atoms and one Br atom.
  • Start at Br.
  • Follow only the bond touching Br.
  • Which carbon is at the other endpoint?

Your decision

Choose C1 or C2 and point to the bond used as evidence.

3 · InvestigationOpen

Investigation

Investigation 1

Read the endpoints

Read these words and symbols first

directly bonded
The same bond line has the two named atoms at its endpoints.
Looking close on the page does not count without a bond line.
X
Read X as ‘ex.’
X is a placeholder for one halogen atom.
In CH3-Br, X represents Br.
X is not a new element.
Identify the actual halogen in each structure.

Do

Place a finger on X, cross one bond and stop at the other endpoint.

Structure of a two-carbon molecule. C1 has three hydrogens and a bond to C2. C2 has two hydrogens, a bond to C1 and a bond to Br.

Check

  • One crossed bond from Br ends at C2.
  • Reaching C1 requires a second bond.

Investigation 2

Use the expanded structure

Read these words and symbols first

expanded structural formula
A drawing that shows every atom and every bond used in the problem.
It shows connections
it is still a flat drawing of a three-dimensional molecule.

Do

Count the bonds at each carbon and verify ordinary carbon valency four.

Structure of a two-carbon molecule. C1 has three hydrogens and a bond to C2. C2 has two hydrogens, a bond to C1 and a bond to Br.

Check

  • C1 has three C-H bonds and one C-C bond
  • C2 has two C-H bonds, one C-C bond and one C-Br bond.

Investigation 3

Treat C1 and C2 as labels

Read these words and symbols first

C1, C2, C3
Temporary labels that let us point to different carbon atoms in one drawing.
These labels are not automatically IUPAC position numbers and the digits are not atom counts.

Do

Point to each labelled carbon and say which bonds touch it.

Structure of a two-carbon molecule. C1 has three hydrogens and a bond to C2. C2 has two hydrogens, a bond to C1 and a bond to Br.

Check

Changing a temporary label would change only our reference name, not the molecule's bonds.

Investigation 4

Read atom inventory

Read these words and symbols first

molecular formula
Element symbols and subscripts that count the atoms in one molecule.
It may not show which atom is bonded to which.

Do

Recount both C3H7Br structures from their expanded diagrams.

Three-carbon structure. Its complete non-hydrogen bond list is Br-C1, C1-C2 and C2-C3.
Three-carbon structure. Its complete non-hydrogen bond list is C1-C2, C2-C3 and C2-Br.

Check

Both diagrams contain the same atom inventory.

Investigation 5

Separate inventory from connectivity

Read these words and symbols first

molecular formula
Element symbols and subscripts that count the atoms in one molecule.
It may not show which atom is bonded to which.
expanded structural formula
A drawing that shows every atom and every bond used in the problem.
It shows connections
it is still a flat drawing of a three-dimensional molecule.
connectivity
The record of which atoms are bonded to which neighbours.
Connectivity alone does not give complete three-dimensional shape.
directly bonded
The same bond line has the two named atoms at its endpoints.
Looking close on the page does not count without a bond line.

Do

Trace from Br once in each of the two C3H7Br structures.

Three-carbon structure. Its complete non-hydrogen bond list is Br-C1, C1-C2 and C2-C3.
Three-carbon structure. Its complete non-hydrogen bond list is C1-C2, C2-C3 and C2-Br.

Check

  • The first structure contains Br-C1
  • the second contains Br-C2, although both recount to C3H7Br.
4 · MeaningOpen

Build the meaning

Meaning 1

Read the endpoints

Two atoms are directly bonded only when one bond line ends at both atoms.

Check

  • One crossed bond from Br ends at C2.
  • Reaching C1 requires a second bond.

Meaning 2

Use the expanded structure

An expanded structural formula displays every atom and bond needed for this decision.

Check

  • C1 has three C-H bonds and one C-C bond
  • C2 has two C-H bonds, one C-C bond and one C-Br bond.

Meaning 3

Treat C1 and C2 as labels

  • C1 and C2 identify atoms in this problem
  • they are solving labels, not chemical subscripts or final name locants.

Check

Changing a temporary label would change only our reference name, not the molecule's bonds.

Meaning 4

Read atom inventory

  • C3H7Br means three carbon atoms, seven hydrogen atoms and one bromine atom
  • no subscript after Br means one Br.

Check

Both diagrams contain the same atom inventory.

Meaning 5

Separate inventory from connectivity

  • Equal atom inventories do not guarantee equal connectivity
  • a structural formula supplies the bond map.

Check

  • The first structure contains Br-C1
  • the second contains Br-C2, although both recount to C3H7Br.
5 · ExamplesOpen

Worked examples

Example 1 of 4

One ordinary C-Br bond

Which carbon is directly bonded to Br?

Structure of a two-carbon molecule. C1 has three hydrogens and a bond to C2. C2 has two hydrogens, a bond to C1 and a bond to Br.

Key evidence

  • One displayed line touches Br.
  • Its other endpoint is C2.

One lawful move at a time

One move at a time

Step 1 of 2
Action
Start at Br.
Why
The question names Br.
Result
The C-Br bond is selected.

Answer

Br is directly bonded to C2.

Independent check

  • Construct the complete non-hydrogen bond list: C1-C2 and C2-Br.
  • Its Br entry confirms C2.

Counter-pattern

C1 is in the same molecule but lies two bonds from Br.

6 · QuestionsOpen

Questions

Question 1 of 12

Read the two endpoints of one explicitly drawn carbon-bromine bond.

In the expanded structure, which carbon is directly bonded to Br?

Structure of a two-carbon molecule. C1 has three hydrogens and a bond to C2. C2 has two hydrogens, a bond to C1 and a bond to Br.

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