Chapter 4 Alkanes

A comprehensive set of question-and-answer flashcards covering alkane structure, isomerism, alkyl groups, monocycloalkanes, IUPAC naming rules, and conformational analysis (ethane and cycloalkanes) based on the lecture notes.

What are alkanes and how are their carbon atoms bonded?

Alkanes are hydrocarbons containing only carbon and hydrogen; all carbon atoms are sp3 hybridized and connected by single bonds.

What are cycloalkanes? What are monocyclic alkanes?

Cycloalkanes: alkanes with one or more rings

monocyclic alkanes: have a single ring.

Name the first three acyclic alkanes and give their molecular formulas.

Methane CH4; Ethane C2H6; Propane C3H8.

What is the general formula for acyclic alkanes?

CnH2n+2.

What is the general formula for monocyclic alkanes?

CnH2n.

What are constitutional isomers?

Compounds with the same molecular formula but different connectivity (aka Lewis structures).

What are the two constitutional isomers of C4H10?

n-butane and isobutane (2-methylpropane).

Define an alkyl group

A substituent derived from an alkane by removing one hydrogen

Are all hydrogens in ethane equivalent?

Yes; all six hydrogens in ethane are equivalent.

What does a shorthand structure show and what is omitted for alkanes?

Shows C–C bonds in a zig-zag form; hydrogens and the explicit carbon atoms at bond intersections/endpoints are not drawn.

How do you decide if two hydrogens are equivalent?

If replacing each with a different atom (e.g., Cl) yields the same molecule, the hydrogens are equivalent; otherwise they are nonequivalent.

What is a methyl group and how is its name derived?

A methyl group is -CH3; the name comes from methane with the last two letters “ane” replaced by “yl.”

List some common alkyl groups used as substituents.

Methyl, Ethyl, Propyl, Isopropyl, Isobutyl, Butyl, Sec-Butyl, Tert-Butyl.

Which alkanes have two sets of nonequivalent hydrogens (in terms of naming)?

Propane has two sets of nonequivalent hydrogens (CH3 vs CH2 groups).

In butane, how many sets of nonequivalent hydrogens are there, and what happens if you remove a hydrogen from a CH3 vs a CH2 group?

Butane has two sets of nonequivalent hydrogens; removing from CH3 gives the butyl group, removing from CH2 gives the secondary-butyl group (sec-).

In isobutyl, what hydrogens removal leads to tert-butyl versus isobutyl?

Removing a hydrogen from a CH3 in isobutyl gives isobutyl; removing from the CH leads to tert-butyl (tert-).

What prefixes indicate secondary and tertiary alkyl groups, and how are they typically formatted?

sec- (secondary) and tert- (tertiary); these prefixes are italicized and hyphenated (e.g., sec-butyl, tert-butyl).

How are alkyl groups shown in shorthand (page 19 examples)?

Common shorthand includes sec-butyl, methyl, propyl, ethyl, isopropyl, isobutyl, butyl, tert-butyl.

What is monocyclic alkane naming and the smallest monocyclic alkane?

Monocyclic alkanes are cycloalkanes; smallest is cyclopropane (3-membered ring).

What is the general formula for monocyclic alkanes and what does it imply about hydrogens on the ring?

General formula is CnH2n; each ring carbon bears two hydrogens.

How many constitutionally isomeric monocyclic alkanes exist for C5H10?

Five different monocyclic isomers.

What is the first step in IUPAC naming of open-chain alkanes?

Select the longest continuous carbon chain as the parent chain.

If two chains have the same length, how do you choose the parent chain?

Choose the one with the greater number of substituents.

How do you number the parent chain in open-chain alkanes?

Number from the end that gives the first substituent the lowest possible number.

What do you do if the first substituents are equidistant from both ends of the chain?

Begin numbering from the end closest to the second substituent.

What if all substituents are equidistant from both ends?

Use the alphabetical order of substituent names to decide the numbering.

How do you cite substituents in IUPAC naming?

Place substituents in alphabetical order (ignore prefixes di, tri, sec-, tert-) before the name of the parent chain.

What is the rule for cycloalkane naming when there are substituents?

Use the prefix cyclo- before the name of the open-chain alkane with the same number of carbons; assign substituents to give the lowest set of locants and list them alphabetically.

When would you treat the ring as a substituent rather than the side chain?

If the substituent has more carbon atoms than the ring, treat the ring as the substituent in the side-chain naming.

What is a Newman projection and what do the center and circle represent?

A projection along the C–C bond; the center represents the front carbon, the circle represents the back carbon; bonds to the back carbon are drawn to the edge of the circle.

What is the dihedral angle in a Newman projection?

The angle between the projection of a C–H bond on the front carbon and a C–H bond on the back carbon, denoted by θ.

Which conformations of ethane are more stable and what is the approximate energy difference?

The staggered conformation is more stable than the eclipsed one; energy difference is about 3 kcal/mol (12 kJ/mol).

What happens to energy when ethane rotates from eclipsed to staggered and back?

From eclipsed to staggered, energy is released (~3 kcal/mol) as the molecule relaxes; from staggered to eclipsed, energy is gained (strained) (~3 kcal/mol).