Intro to Hydrocarbons - Online Tutor, Practice Problems & Exam Prep

Hydrocarbons are organic compounds made of carbon and hydrogen. They include alkanes (C_nH_2n+2), alkenes (C_nH_2n), alkynes (C_nH_2n-2), cycloalkanes (C_nH_2n), and aromatic compounds like benzene (C₆H₆). Saturated hydrocarbons have only single bonds, maximizing hydrogen atoms, while unsaturated ones contain double or triple bonds, reducing hydrogen count. Bond rotation varies: single bonds allow free rotation, while double and triple bonds restrict it, leading to different spatial orientations.

Intro to Hydrocarbons Concept 1

Now, what carbon hydrocarbons are the simplest organic compounds composed solely of carbons and hydrogens. Here in this list, we're going to go over different types of hydrocarbons. So, if we start with the first row, we have alkanes. Alkanes are just carbons that are single bonded to one another. Now if we pay attention, remember, each one of these bonds connects two carbons together, carbon must make 4 bonds. If we look at the two carbons in the middle, they each have two hydrogens. Now, focus on either one of these carbons, and you would see that they are connected to 4 surrounding atoms, meaning that their hybridization is sp3. Their generic formula, if we were to look here, we'd have 4 carbons, and you would have 10 hydrogens. And that's because the generic formula for an alkane is CnH2n+2, where n is the number of carbon atoms. So if we said that we need an alkane that has 5 carbons, what is its molecular formula? It'd be C₅H₁₂. Now, alkenes is when you have two carbons double bonded to each other; they still need to make 4 bonds, they're making 3, so they each have one hydrogen that's invisible. Notice that the number of hydrogens decreased as we added that pi bond between the carbons. These carbons here are connected to three surrounding atoms, so they are sp2 in terms of hybridization. Each of these double-bonded carbons lost a hydrogen, so we lost two hydrogens overall. So our generic formula has changed to CnH2n. We've dropped the plus 2 because we lost those two hydrogens. Alkynes are triple bonded carbons, so there's one here and here. They're making four bonds right now, so they don't need any extra hydrogens on them. These two sp carbons are sp hybridized because they are connected to two surrounding atoms. And think about it, we just lost another two hydrogens, so the generic formula becomes CnH2n-2. Cycloalkanes are just alkanes that are in rings. So they're all still single bonded. In this ring, we have carbons all around. Each of these carbons has two hydrogens on them, right? So if we think about it, focus on one of them. Let's focus on this one right here. It is connected to four surrounding atoms, so it is sp3 hybridized. Here if we look, we have five carbons for ten hydrogens, so the ratio is there's double the amount of hydrogens to carbon. So that means the formula will be CnH2n. So a Cycloalkane has the same generic formula as an alkene. We have to lose two hydrogens in order to enclose this alkane into a ring. Finally, aromatic refers to a Benzene ring. A Benzene ring is six carbons in a ring with alternating double bonds. Each of these carbons has one hydrogen. So a Benzene has a formula of C₆H₆. Another way of showing it is like this. This is changing it into more of a skeletal formula. Each of those carbons are double-bonded, they're each connected to three surrounding atoms, so they are sp2 hybridized. And here it's a one-to-one relationship between Carbon and Hydrogen since it's C₆H₆. So the generic formula is C_NH_N. Alright. So these encompass all the different types of hydrocarbons that you may encounter when dealing with deeper types of ideas in organic chemistry. But remember, essentially they're all just compounds with carbons and hydrogens. Here some of them have single bonds, some of them have double bonds, some of them have triple bonds or are in rings. Okay. But they're all hydrocarbons at the end of the day.

Intro to Hydrocarbons Example 1

So in this example question, it says, classify each of the following hydrocarbons as an alkane, alkene, or alkyne. So remember, an alkane is when we have carbons just single bonded to one another. If we take a look at the options, remember every line connects 2 carbons to each other. In this option B, all of them are single bonded to one another, so they're all alkanes. Well, it is an alkane. This is the only alkane that we have. An alkene is when we have carbons double bonded to one another. These 2 carbons here are double bonded to each other, and as long as 2 carbons are double bonded to each other, it is an alkene. These 2 carbons are double bonded to each other, so this is also an alkene. An alkyne is when we have 2 carbons triple bonded to one another. We have a carbon here, and a carbon here. So option C would represent an alkyne. This is how we describe our options A, B, C, and D for the following example question.

Saturated and Unsaturated Hydrocarbons Concept 2

Now with hydrocarbons, we now talk about saturated versus unsaturated. A saturated hydrocarbon means that all the bonds are single bonds. All the carbons connected to each other have single bonds, which allows each carbon to have the maximum possible number of hydrogen atoms. Consider the image here, where the one on the left is a saturated hydrocarbon. Those three carbons have to make four bonds each, and to achieve this, we'd have to add hydrogens to them.

An unsaturated hydrocarbon means that there is at least one double bond or triple bond, implying that we won't be able to have a maximum number of hydrogens. Taking a look at this double bond here, what effect will that have? The carbon on the left is single bonded, so it still has its maximum number of three hydrogens. However, because of that double bond there, remember each carbon needs to make four bonds. This carbon here is already making three bonds, so it only needs one hydrogen. The same applies to this carbon, which also needs one additional hydrogen. What's the difference? Well, these two carbons, the one here had two hydrogens, but with the inclusion of a double bond, now it only has one. This carbon here has three hydrogens, but the inclusion of the double bond means now it has only two. So, adding a Pi bond or a double bond means that we're going to have to forfeit one hydrogen atom per carbon. And if we were to make a triple bond, again we'd need to forfeit one hydrogen atom between those carbons. & Therefore, this illustrates the difference between a saturated hydrocarbon, where there are no double bonds or triple bonds, so you have the maximum of hydrogen atoms on carbons, and an unsaturated hydrocarbon, where now double bonds and triple bonds decrease the total number of H atoms possible.

Saturated and Unsaturated Hydrocarbons Example 2

In this example question, it says, classify the following hydrocarbons as saturated or unsaturated. Remember, a saturated hydrocarbon possesses only single bonds between its carbons. In this image, a and c would represent saturated hydrocarbons. Every line connects carbons together. All these carbons are single bonded to one another, meaning they'll have a maximum number of hydrogen atoms attached to each of them. Unsaturated would be these 2. Unsaturated hydrocarbons have a double bond or a triple bond present. This decreases the maximum number of H atoms that can connect to a potential carbon. Remember, carbon must make 4 bonds. Adding those Pi bonds decreases that number. Alright. So a and c would be saturated, b and d would be unsaturated.

Write the molecular formula for an alkane with 5 C atoms.

• A. C₅H₁₂
• B. C₅H₁₀
• C. C₅H₁₄
• D. C₅H₈

Write the molecular formula for an alkyne with 4 C atoms.

• A. C₄H₈
• B. C₄H₁₀
• C. C₄H₆
• D. C₄H₄

Which of the following molecular formulas might indicate an alkene?

• A. C₇H₁₆
• B. C₆H₁₂
• C. C₅H₈
• D. C₄H₁₀

Bond Rotation and Spatial Orientation Concept 3

So in our continued discussion of hydrocarbons, we're now going to take a look at bond rotation and spatial orientation. Here we're going to say carbon-carbon bonds in alkanes can rotate freely, and that's because of the presence of a single bond. So here, this is a carbon and this is another carbon, and we rotate around this single bond. Now by rotating that, we're going to keep one side stationary, meaning we're not going to move it. We're going to keep this side still, so it matches up with this side. And we're going to rotate the other part, the right side. When I'm rotating, this red ball is up here, so when I rotate, it'll rotate down to where the blue is. So there it goes right there. And then when I rotate that down, this blue comes to where this part is, so there goes the blue part right there. And then that green thing has to shift up to this, to where the red was, so there's the green part right up here.

How would this look like in terms of a skeletal formula? So again, there's free rotation around the single bond, so we rotate, we keep this side stationary, so it stays where it is. Everything else is spinning. This O would rotate down to where the H is, so that's why it's here. This H would rotate to where this methyl group is, and then this, this CH₃ group here would rotate up here.

Now, if we have a double bond, which is in alkenes, we cannot rotate. There's no free rotation around a pi bond, a double bond. This leads to two different spatial orientations and therefore two different compounds. These two carbons are double bonded, so we can't rotate. That means that these two blues are locked on the same side, and these two greens are locked on the same side. So it would not be equal to this molecule here, where the blues are opposites of each other and the greens are on opposite sides of each other. If we show this as a skeletal formula, these two H's are locked on the same side, but here they're on opposite sides. There's no free rotation, so these will represent two different compounds.

So just keep in mind when we're dealing with double bonds and also triple bonds, there's no free rotation around that bond. Things are locked where they are.

Bond Rotation and Spatial Orientation Example 3

Here it says determine if the two structures below are the same or different compounds. Well, both compounds have 2 carbons double bonded to each other. Remember, there is no free rotation around that double bond, so things are stuck where they are. The one on the left has an H and an F next to each other, but the one on the right has 2 H's instead next to each other. The one on the left has a Br and an H next to each other, but this one here has a Br and an F next to each other. We have different groups next to one another in both molecules or both compounds, and we can't rotate around that double bond so that they're the same. Because of this, they would represent different compounds. So here, our answer would be option b.

Which of the following is not a valid bond rotation?

• A.
• B.
• C.
• D.

Here’s what students ask on this topic:

What is the difference between alkanes, alkenes, and alkynes?

Alkanes, alkenes, and alkynes are all types of hydrocarbons, but they differ in the types of bonds between carbon atoms. Alkanes have only single bonds between carbon atoms, following the general formula C_nH_2n+2. Alkenes contain at least one double bond between carbon atoms, with the formula C_nH_2n. Alkynes have at least one triple bond between carbon atoms, and their formula is C_nH_2n-2. These differences in bonding affect their chemical properties and reactivity.

What is the general formula for cycloalkanes?

Cycloalkanes are hydrocarbons that form ring structures with single bonds between carbon atoms. The general formula for cycloalkanes is C_nH_2n. This formula is similar to that of alkenes because forming a ring requires the loss of two hydrogen atoms compared to a straight-chain alkane.

How does bond rotation differ between alkanes and alkenes?

In alkanes, the single bonds between carbon atoms allow for free rotation, meaning the atoms can rotate around the bond axis without restriction. This is not the case for alkenes, which have double bonds. The presence of a π bond in alkenes restricts rotation, locking the atoms in place and leading to different spatial orientations, known as cis and trans isomers.

What is the significance of hybridization in hydrocarbons?

Hybridization in hydrocarbons refers to the mixing of atomic orbitals to form new hybrid orbitals. In alkanes, carbon atoms are sp³ hybridized, forming four single bonds. In alkenes, carbon atoms involved in double bonds are sp² hybridized, forming three bonds (one double and two single). In alkynes, carbon atoms in triple bonds are sp hybridized, forming two bonds (one triple and one single). Hybridization affects the geometry and reactivity of the molecules.

What is the difference between saturated and unsaturated hydrocarbons?

Saturated hydrocarbons contain only single bonds between carbon atoms, allowing each carbon to bond with the maximum number of hydrogen atoms. Examples include alkanes. Unsaturated hydrocarbons have one or more double or triple bonds, reducing the number of hydrogen atoms. Examples include alkenes (with double bonds) and alkynes (with triple bonds). The presence of multiple bonds affects the chemical properties and reactivity of the molecules.