Organic Chemistry I: Hydrocarbons

Questions and Answers

What is the term used to describe the arrangement of atoms that can be converted into one another by rotation about carbon-carbon single bonds?

Eclipsed conformations

Newman projections

Torsional strain

Conformations (correct)

What is the most stable conformation of ethane molecules?

Staggered conformation (correct)

Skew conformation

Newman projection

Eclipsed conformation

What is the energy required to rotate the ethane molecule about the carbon-carbon bond?

Torsional strain

Rotational barrier

Conformational energy

Torsional energy (correct)

What is the main reason for the rotational barrier in propane and ethane?

Torsional strain Signup and view all the answers

What is the difference between n-butane and isobutane?

Physical and chemical properties Signup and view all the answers

What is the term used to describe the representations of the kind shown in the figure?

Newman projections Signup and view all the answers

What is the arrangement of atoms in which the hydrogens exactly oppose each other?

Eclipsed conformation Signup and view all the answers

What is the potential energy of the molecule at the eclipsed conformation?

Reaches a maximum Signup and view all the answers

What are the two main elements that make up hydrocarbons?

Hydrogen and carbon Signup and view all the answers

What is the shape of the bonding orbitals in methane?

Tetrahedral Signup and view all the answers

What is the simplest member of the alkane family?

Methane Signup and view all the answers

What type of bond is formed between the carbon atoms in ethane?

σ bond Signup and view all the answers

What is the arrangement of the hydrogen nuclei in methane?

Tetrahedral Signup and view all the answers

What is the effect of the tetrahedral arrangement of the bonding orbitals in methane?

Strengthen the bonds Signup and view all the answers

What is the characteristic of the carbon-carbon bond in ethane?

Free rotation is possible Signup and view all the answers

What is the classification of hydrocarbons based on their structure?

Aliphatic and aromatic Signup and view all the answers

What is the difference in stability between the anti-conformation and the gauche conformation in n-butane?

0.8 kcal/mole Signup and view all the answers

What is the dihedral angle in the anti-conformation of n-butane?

180o Signup and view all the answers

What is the reason for the higher energy of the gauche conformation in n-butane?

Van der Waals repulsion Signup and view all the answers

What is the general formula for members of the homologous series of alkanes?

CnH2n+2 Signup and view all the answers

What is the constant difference between successive members of the homologous series of alkanes?

CH2 Signup and view all the answers

How many hydrogen atoms are present in an alkane with n carbon atoms?

2n+2 Signup and view all the answers

What is the reason for the lower energy of the staggered conformations in ethane and n-butane?

Lower torsional energies Signup and view all the answers

What is the term used to describe the repulsion between the methyl groups in the gauche conformation of n-butane?

Van der Waals repulsion Signup and view all the answers

What is the general formula for an alkane?

CnH2n+2 Signup and view all the answers

What is the name of the alkane with five carbon atoms?

Pentane Signup and view all the answers

What is the naming convention for alkyl groups?

Drop -ane and replace it with -yl Signup and view all the answers

What is the general formula for an alkyl group?

CnH2n+1 Signup and view all the answers

What is the reason for the use of prefixes in naming alkanes?

To distinguish between isomers Signup and view all the answers

What is the name of the CH3 group?

Methyl Signup and view all the answers

How many propyl groups are there?

Two Signup and view all the answers

What is the result of increasing the number of atoms in an alkane?

The number of possible arrangements increases Signup and view all the answers

What is the correct way to name an alkane with multiple alkyl groups?

In order of increasing size, then alphabetical order Signup and view all the answers

What is the classification of a carbon atom attached to three other carbon atoms?

Tertiary (3o) Signup and view all the answers

What is the reason for the increase in boiling points and melting points of alkanes as the number of carbons increases?

The intermolecular forces increase Signup and view all the answers

What is the correct way to number the parent carbon chain in an alkane?

Start at whichever end results in the use of the lowest numbers Signup and view all the answers

What is the prefix used to indicate the presence of two identical alkyl groups?

Di- Signup and view all the answers

What is the correct name for the alkane with the structure shown in (I)?

Methylpropane Signup and view all the answers

What is the classification of a hydrogen atom attached to a tertiary carbon atom?

The same as the carbon atom to which it is attached Signup and view all the answers

What is the purpose of the numbering system in naming alkanes?

To indicate the location of the alkyl groups Signup and view all the answers

Study Notes

Hydrocarbons

Hydrocarbons are organic compounds containing only hydrogen and carbon.
They are divided into two main classes: aliphatic and aromatic.
Aliphatic hydrocarbons are further divided into families: alkanes, alkenes, alkynes, and their cyclic analogs.

Alkanes

The simplest alkane is methane (CH4), with a tetrahedral arrangement of bonding orbitals.
Each carbon atom is bonded to four other atoms, with its bonding orbitals (sp3 orbitals) directed to the corners of a tetrahedron.
The next largest alkane is ethane (C2H6), with a carbon-carbon bond arising from the overlap of two sp3 orbitals.

Conformations of Ethane

Different arrangements of atoms that can be converted into one another by rotation about carbon-carbon single bonds are called conformations.
The eclipsed conformation (I) has a higher energy than the staggered conformation (II), due to torsional strain.
The staggered conformation is the most stable, with a minimum potential energy.

Higher Alkanes

Propane (C3H8) and butane (C4H10) are larger alkanes, with increasing rotational barriers due to torsional strain.
Butane has two possible structures: n-butane (a four-carbon chain) and isobutane (a three-carbon chain with a one-carbon branch).
These two structures are different compounds, with distinct physical and chemical properties.

Conformations of n-Butane

The anti-conformation (I) has a dihedral angle of 180°, with the methyl groups as far apart as possible.
The gauche conformations (II and III) have a dihedral angle of 60°, with the methyl groups closer together.
The anti-conformation is more stable (by 0.8 kcal/mole) than the gauche conformations, due to van der Waals repulsion.

Homologous Series

The family of alkanes forms a homologous series, with each member differing from the next by a constant amount (CH2).
The general formula for alkanes is CnH2n+2, with the number of hydrogen atoms equal to two more than twice the number of carbon atoms.

Nomenclature

The names of alkanes are derived from the Greek or Latin prefix for the number of carbon atoms in the alkane.
Except for the first four members, the name is derived from the prefix and the suffix "-ane".
Isomers are distinguished by the use of prefixes, such as n- and iso-.

Alkyl Groups

Alkyl groups are named by dropping "-ane" from the name of the corresponding alkane and replacing it with "-yl".
The general formula for an alkyl group is CnH2n+1.
Alkyl groups can be isomeric, such as the two propyl groups.

Physical Properties

The boiling points and melting points of alkanes rise as the number of carbon atoms increases.
This is due to the increasing intermolecular forces between larger molecules.

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Description

This quiz covers the basics of hydrocarbons, including their structure, classification, and types such as aliphatic and aromatic hydrocarbons. It also touches on the different families of aliphatic hydrocarbons, including alkanes, alkenes, and alkynes.